Fix a bug where requesting a review would fail with an erroneous message.
Fix a bug where requesting a review would fail with an erroneous message.

We were figuring out the set of commits to include in a review by
listing the commits between the head of the target ref and the head of
the source ref. However, this only works if the source ref is a
fast-forward of the target ref.

This commit changes it so that we use the merge-base of the target and
source refs as the starting point instead of the target ref.

google/git-appraise