Teach bash completion about git-shortlog

Teach bash completion about git-shortlog

We've had completion for git-log for quite some time, but just
today I noticed we don't have it for the new builtin shortlog
that runs git-log internally.  This is indeed a handy thing to
have completion for, especially when your branch names are of
the Very-Very-Long-and-Hard/To-Type/Variety/That-Some-Use.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>