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 pull request #496 from mikedld/show-committer-of-not-author

Show committer if it differs from author