branch: allow -f with -m and -d

-f/--force is the standard way to force an action, and is used by branch
for the recreation of existing branches, but not for deleting unmerged
branches nor for renaming to an existing branch.

Make "-m -f" equivalent to "-M" and "-d -f" equivalent to" -D", i.e.
allow -f/--force to be used with -m/-d also.

For the list modes, "-f" is simply ignored.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>