This is where portable git stores the global configuration which we can
use to adhere to it even though git isn't quite installed on the system.

This makes the API for commiting or discarding changes the same as for
references.

This changes the get_entry() method to return a refcounted version of
the config entry, which you have to free when you're done.

This allows us to avoid freeing the memory in which the entry is stored
on a refresh, which may happen at any time for a live config.

For this reason, get_string() has been forbidden on live configs and a
new function get_string_buf() has been added, which stores the string in
a git_buf which the user then owns.

The functions which parse the string value takea advantage of the
borrowing to parse safely and then release the entry.

And decrease extra reload checks of config data.

This fixes `git_submodule_sync` to correctly update the remote URL
of the default branch of the submodule along with the URL in the
parent repository config (i.e. match core Git's behavior).

Also move some useful helper logic from the submodule code into
a shared config API `git_config__update_entry` that can either set
or delete an entry with constraints like not overwriting or not
creating a new entry.  I used that helper to update a couple other
places in the code.

Again, we already did this internally, so simply remove the conversions.

This adds `git_config__lookup_entry` which will look up a key in
a config and return either the entry or NULL if the key was not
present.  Optionally, it can either suppress all errors or can
return them (although not finding the key is not an error for this
function).  Unlike other accessors, this does not normalize the
config key string, so it must only be used when the key is known
to be in normalized form (i.e. all lower-case before the first dot
and after the last dot, with no invalid characters).

This also adds three high-level helper functions to look up config
values with no errors and a fallback value.  The three functions
are for string, bool, and int values, and will resort to the
fallback value for any error that arises.  They are:

* `git_config__get_string_force`
* `git_config__get_bool_force`
* `git_config__get_int_force`

None of them normalize the config `key` either, so they can only
be used for internal cases where the key is known to be in normal
format.

When a repository is transferred from one file system to another,
many of the config settings that represent the properties of the
file system may be wrong.  This adds a new public API that will
refresh the config settings of the repository to account for the
change of file system.  This doesn't do a full "reinitialize" and
operates on a existing git_repository object refreshing the config
when done.

This commit then makes use of the new API in clar as each test
repository is set up.

This commit also has a number of other clar test fixes where we
were making assumptions about the type of filesystem, either based
on outdated config data or based on the OS instead of the FS.

Build it on top of the normal iterator instead, which lets use re-use
a lot of code.