The variable '.path' of the refdb_fs_backend struct becomes
confusing regarding the introduction of the git commondir. It
does not immediatly become obvious what it should point to.

Fix this problem by renaming the variable to `gitpath`,
clarifying that it acutally points to the `.git` directory of the
repository, in contrast to the commonpath directory, which points
to the directory containing shared objects like references and
the object store.