When we expect a checkout operation to fail, but it succeeds, we
actually do not want to see the error messages that were generated in
the meantime for errors that were handled gracefully by the code (e.g.
when an object could not be found in a pack: in this case, the next
backend would have been given a chance to look up the object, and
probably would have found it because the checkout succeeded, after all).

Which means that in the specific case of `cl_git_fail()`, we actually
want to clear the global error state _after_ evaluating the command: we
know that any still-available error would be bogus, seeing as the
command succeeded (unexpectedly).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

To decide whether a config file has changed, we always hash its
complete contents. This is unnecessarily expensive, as
well-behaved filesystems will always update stat information for
files which have changed. So before computing the hash, we should
first check whether the stat info has actually changed for either
the configuration file or any of its includes. This avoids having
to re-read the configuration file and its includes every time
when we check whether it's been modified.

Tracing the for-each-ref example previous to this commit, one can
see that we repeatedly re-open both the repo configuration as
well as the global configuration:

	$ strace lg2 for-each-ref |& grep config
	access("/home/pks/.gitconfig", F_OK)    = -1 ENOENT (No such file or directory)
	access("/home/pks/.config/git/config", F_OK) = 0
	access("/etc/gitconfig", F_OK)          = -1 ENOENT (No such file or directory)
	stat("/tmp/repo/.git/config", {st_mode=S_IFREG|0644, st_size=92, ...}) = 0
	access("/tmp/repo/.git/config", F_OK)   = 0
	stat("/tmp/repo/.git/config", {st_mode=S_IFREG|0644, st_size=92, ...}) = 0
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	stat("/home/pks/.gitconfig", 0x7ffd15c05290) = -1 ENOENT (No such file or directory)
	access("/home/pks/.gitconfig", F_OK)    = -1 ENOENT (No such file or directory)
	stat("/home/pks/.config/git/config", {st_mode=S_IFREG|0644, st_size=1154, ...}) = 0
	access("/home/pks/.config/git/config", F_OK) = 0
	stat("/home/pks/.config/git/config", {st_mode=S_IFREG|0644, st_size=1154, ...}) = 0
	open("/home/pks/.config/git/config", O_RDONLY|O_CLOEXEC) = 3
	stat("/tmp/repo/.git/config", {st_mode=S_IFREG|0644, st_size=92, ...}) = 0
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	stat("/home/pks/.gitconfig", 0x7ffd15c051f0) = -1 ENOENT (No such file or directory)
	stat("/home/pks/.config/git/config", {st_mode=S_IFREG|0644, st_size=1154, ...}) = 0
	open("/home/pks/.config/git/config", O_RDONLY|O_CLOEXEC) = 3
	stat("/tmp/repo/.git/config", {st_mode=S_IFREG|0644, st_size=92, ...}) = 0
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	stat("/home/pks/.gitconfig", 0x7ffd15c05090) = -1 ENOENT (No such file or directory)
	stat("/home/pks/.config/git/config", {st_mode=S_IFREG|0644, st_size=1154, ...}) = 0
	open("/home/pks/.config/git/config", O_RDONLY|O_CLOEXEC) = 3
	stat("/tmp/repo/.git/config", {st_mode=S_IFREG|0644, st_size=92, ...}) = 0
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	stat("/home/pks/.gitconfig", 0x7ffd15c05090) = -1 ENOENT (No such file or directory)
	stat("/home/pks/.config/git/config", {st_mode=S_IFREG|0644, st_size=1154, ...}) = 0
	open("/home/pks/.config/git/config", O_RDONLY|O_CLOEXEC) = 3
	stat("/tmp/repo/.git/config", {st_mode=S_IFREG|0644, st_size=92, ...}) = 0
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	stat("/home/pks/.gitconfig", 0x7ffd15c05090) = -1 ENOENT (No such file or directory)
	stat("/home/pks/.config/git/config", {st_mode=S_IFREG|0644, st_size=1154, ...}) = 0
	open("/home/pks/.config/git/config", O_RDONLY|O_CLOEXEC) = 3

With the change, we only do stats for those files and open them a
single time, only:

	$ strace lg2 for-each-ref |& grep config
	access("/home/pks/.gitconfig", F_OK)    = -1 ENOENT (No such file or directory)
	access("/home/pks/.config/git/config", F_OK) = 0
	access("/etc/gitconfig", F_OK)          = -1 ENOENT (No such file or directory)
	stat("/tmp/repo/.git/config", {st_mode=S_IFREG|0644, st_size=92, ...}) = 0
	access("/tmp/repo/.git/config", F_OK)   = 0
	stat("/tmp/repo/.git/config", {st_mode=S_IFREG|0644, st_size=92, ...}) = 0
	stat("/tmp/repo/.git/config", {st_mode=S_IFREG|0644, st_size=92, ...}) = 0
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	stat("/home/pks/.gitconfig", 0x7ffe70540d20) = -1 ENOENT (No such file or directory)
	access("/home/pks/.gitconfig", F_OK)    = -1 ENOENT (No such file or directory)
	stat("/home/pks/.config/git/config", {st_mode=S_IFREG|0644, st_size=1154, ...}) = 0
	access("/home/pks/.config/git/config", F_OK) = 0
	stat("/home/pks/.config/git/config", {st_mode=S_IFREG|0644, st_size=1154, ...}) = 0
	stat("/home/pks/.config/git/config", {st_mode=S_IFREG|0644, st_size=1154, ...}) = 0
	open("/home/pks/.config/git/config", O_RDONLY|O_CLOEXEC) = 3
	stat("/tmp/repo/.git/config", {st_mode=S_IFREG|0644, st_size=92, ...}) = 0
	stat("/home/pks/.gitconfig", 0x7ffe70540ca0) = -1 ENOENT (No such file or directory)
	stat("/home/pks/.gitconfig", 0x7ffe70540c80) = -1 ENOENT (No such file or directory)
	stat("/home/pks/.config/git/config", {st_mode=S_IFREG|0644, st_size=1154, ...}) = 0
	stat("/tmp/repo/.git/config", {st_mode=S_IFREG|0644, st_size=92, ...}) = 0
	stat("/home/pks/.gitconfig", 0x7ffe70540b40) = -1 ENOENT (No such file or directory)
	stat("/home/pks/.gitconfig", 0x7ffe70540b20) = -1 ENOENT (No such file or directory)
	stat("/home/pks/.config/git/config", {st_mode=S_IFREG|0644, st_size=1154, ...}) = 0
	stat("/tmp/repo/.git/config", {st_mode=S_IFREG|0644, st_size=92, ...}) = 0
	stat("/home/pks/.gitconfig", 0x7ffe70540b40) = -1 ENOENT (No such file or directory)
	stat("/home/pks/.gitconfig", 0x7ffe70540b20) = -1 ENOENT (No such file or directory)
	stat("/home/pks/.config/git/config", {st_mode=S_IFREG|0644, st_size=1154, ...}) = 0
	stat("/tmp/repo/.git/config", {st_mode=S_IFREG|0644, st_size=92, ...}) = 0
	stat("/home/pks/.gitconfig", 0x7ffe70540b40) = -1 ENOENT (No such file or directory)
	stat("/home/pks/.gitconfig", 0x7ffe70540b20) = -1 ENOENT (No such file or directory)
	stat("/home/pks/.config/git/config", {st_mode=S_IFREG|0644, st_size=1154, ...}) = 0

The following benchmark has been performed with and without the
stat cache in a best-of-ten run:

```

int lg2_repro(git_repository *repo, int argc, char **argv)
{
	git_config *cfg;
	int32_t dummy;
	int i;

	UNUSED(argc);
	UNUSED(argv);

	check_lg2(git_repository_config(&cfg, repo),
			"Could not obtain config", NULL);

	for (i = 1; i < 100000; ++i)
		git_config_get_int32(&dummy, cfg, "foo.bar");

	git_config_free(cfg);
	return 0;
}
```

Without stat cache:

	$ time lg2 repro
	real    0m1.528s
	user    0m0.568s
	sys     0m0.944s

With stat cache:

	$ time lg2 repro
	real    0m0.526s
	user    0m0.268s
	sys     0m0.258s

This benchmark shows a nearly three-fold performance improvement.

This change requires that we check our configuration stress tests
as we're now in fact becoming more racy. If somebody is writing a
configuration file at nearly the same time (there is a window of
100ns on Windows-based systems), then it might be that we realize
that this file has actually changed and thus may not re-read it.
This will only happen if either an external process is rewriting
the configuration file or if the same process has multiple
`git_config` structures pointing to the same time, where one of
both is being used to write and the other one is used to read
values.

Prefix all the calls to the the regexec family of functions with `p_`.
This allows us to swap out all the regular expression functions with our
own implementation.  Move the declarations to `posix_regex.h` for
simpler inclusion.

Move to the `git_error` name in the internal API for error-related
functions.

Provide more detailed messages when conditions pass or fail
unexpectedly.  In particular, this provides the error messages when a
test fails with a different error code than was expected.

The function `cl_git_thread_check()` is defined as static. As the
function is defined in a header file which is included by our
tests, this can result in warnings for every test file where
`cl_git_thread_check` is never used.

Fix the issue by marking it as inline instead.