Rewrite `fetchgitLocal`

For practical purposes, here are the changes in behavior:
 - When fetching from a subdirectory of a repo, do not rebuild because of
   changes elsewhere in the repo
 - Fetch (not-ignored) untracked files too

It does this by letting git hash and export the directory in question,
which I believes makes for a cleaner implementation than the ad-hoc copying
and hashing that was there before.