Merge branch 'ar/diff-index-merge-base-fix'

"git diff --merge-base X other args..." insisted that X must be a
commit and errored out when given an annotated tag that peels to a
commit, but we only need it to be a committish.  This has been
corrected.

* ar/diff-index-merge-base-fix:
  diff: fix --merge-base with annotated tags

diff --cc diff-lib.c
Simple merge