fix git-annex repair false positive

fix git-annex repair false positive

Avoid treating refs/annex/last-index or other refs that are not commit
objects as evidence of repository corruption.

The repair code checks to find bad refs by trying to run `git log` on
them, and assumes that no output means something is broken.  But git log
on a tree object is empty.

This was worth fixing generally, not as a special case, since it's certainly
possible that other things store tree or other objects in refs.

Sponsored-by: Max Thoursie on Patreon