On Windows, we need to enforce MAX_PATH for loose references and their
reflogs.  Ensure that any path - including the lock file - would fit
within the 260 character maximum.

We do not honor core.longpaths for loose reference files or reflogs.
core.longpaths only applies to paths in the working directory.

As git_reference__name will reallocate storage to account for longer
names (it's actually allocator-dependent), it will cause all existing
pointers to the old object to become dangling, as they now point to
freed memory.

Fix the issue by renaming to a more descriptive name, and pass a pointer
to the actual reference that can safely be invalidated if the realloc
succeeds.