Untangle git_futils_mkdir from git_futils_mkdir_ext - the latter
assumes that we own everything beneath the base, as if it were
being called with a base of the repository or working directory,
and is tailored towards checkout and ensuring that there is no
bogosity beneath the base that must be cleaned up.

This is (at best) slow and (at worst) unsafe in the larger context
of a filesystem where we do not own things and cannot do things like
unlink symlinks that are in our way.

The config is not owned by the transaction, so please don’t free it.

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

This lock/unlock pair allows for the cller to lock a configuration file
to avoid concurrent operations.

It also allows for a transactional approach to updating a configuration
file. If multiple updates must be made atomically, they can be done
while the config is locked.

When a configuration file is locked, any updates made to it will be done
to the in-memory copy of the file. This allows for multiple updates to
happen while we hold the lock, preventing races during complex
config-file manipulation.

If a multivar exists within two sections (of the same name)
then they should both be updated in a `set_multivar`.  Ensure
that this is the case.

Add a test that exposes a bug in config_write.
It is valid to have multiple separate headers for the same config section, but
config_write will exit after finding the first matching section in certain
situations.
This test proves that config_write will duplicate a variable that already
exists instead of overwriting it if the variable is defined under a duplicate
section header.