puppet-merge: warn if multiple committers

puppet-merge makes it easy to merge a change without intending to, because its
diff output is verbose, causing antecedent patches to scroll past. This patch
makes puppet-merge print a summary of commiter names and commit subjects. When
the candidate revision range contains commits by more than one committer,
puppet-merge will also print a warning.

Change-Id: I3f0062fcc95ee856b18198c20e6cd226602d308a