Allow "git shortlog" to group by committer information

In some situations you may want to group the commits not by author,
but by committer instead.

For example, when I just wanted to look up what I'm still missing from
linux-next in the current merge window, I don't care so much about who
wrote a patch, as what git tree it came from, which generally boils
down to "who committed it".

So make git shortlog take a "-c" or "--committer" option to switch
grouping to that.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>