bash completion: fix completion issues with fetch, pull, and push

bash completion: fix completion issues with fetch, pull, and push

Sverre Rabbelier noticed a completion issue with push:

 $ git push ori<tab>
 git push origin

 $ git push -f ori<tab>
 git push -f origin/

Markus Heidelberg pointed out that the issue extends to fetch and pull.

The reason is that the current code naively assumes that if
COMP_CWORD=2, it should complete a remote name, otherwise it should
complete a refspec. This assumption fails if there are any --options.

This patch fixes that issue by instead scanning COMP_CWORDS to see if
the remote has been completed yet (we now assume the first non-dashed
argument is the remote). The new logic is factored into a function,
shared by fetch, pull, and push.

The new function also properly handles '.' as the remote.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>