Add more commit pseudo-headers

Based on the common ones in linux.git and git.git.  To see them, execute
the following on linux.git and git.git:

  $ git log | grep -- -by: | sed -e 's/-by: .*//' | sort | uniq -c |
  sort -r -h | head -n 20

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>