Show committer if it differs from author

It is quite useful sometimes to see that someone else (not you) has pushed
commit to remote. This change adds "C committed on D" phrase right after
"A authored on B" if author and committer are not the same person (judging
from email address).
Merge remote-tracking branch 'mikedld/show-committer-of-not-author'

Show commit committer when they differ from the author of the commit.