Completion/Unix/Command/_git: fix shortlog completer

Currently, __git-shortlog () says that 'git shortlog' can only accept
commits as arguments (probably because the official documentation says
this).  This is entirely untrue: shortlog can accept
commit-range-or-file, just like log can.  Fix the completer by copying
out segments from the __git-log () function.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>