We want a predictable number of initializations in our multithreaded
init test, but we also want to make sure that we have _actually_
initialized `git_libgit2_init` before calling `git_thread_create` (since
it now has a sanity check that `git_libgit2_init` has been called).

Since `git_thread_create` is internal-only, keep this sanity check.
Flip the invocation so that we `git_libgit2_init` before our thread
tests and `git_libgit2_shutdown` again after.

global: synchronize initialization and shutdown with pthreads

Exercise the logic surrounding deinitialization of the libgit2
library as well as repeated concurrent de- and reinitialization.
This tries to catch races and makes sure that it is possible to
reinitialize libgit2 multiple times.

After deinitializing libgit2, we have to make sure to setup
options required for testing. Currently, this only includes
setting up the configuration search path again. Before, this has
been set up once in `tests/main.c`.

The `git_pqueue` struct allows being fixed in its total number of
entries. In this case, we simply throw away items that are
inserted into the priority queue by examining wether the new item
to be inserted has a higher priority than the previous smallest
one.

This feature somewhat contradicts our pqueue implementation in
that it is allowed to not have a comparison function. In fact, we
also fail to check if the comparison function is actually set in
the case where we add a new item into a fully filled fixed-size
pqueue.

As we cannot determine which item is the smallest item in absence
of a comparison function, we fix the `NULL` pointer dereference
by simply dropping all new items which are about to be inserted
into a full fixed-size pqueue.

`xlocale.h` only defines `regcomp_l` if `regex.h` was included as well.

Also change the test cases to actually test `p_regcomp` works with
a multibyte locale.

Improve revision walk preparation logic

This is a convenience function to reverse the contents of a vector and a pqueue
in-place.

The pqueue function is useful in the case where we're treating it as a
LIFO queue.

This code is ported from git.git

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: David Turner <dturner@twopensource.com>