branch: make "-l" a synonym for "--list"

branch: make "-l" a synonym for "--list"

The other "mode" options of git-branch have short-option
aliases that are easy to type (e.g., "-d" and "-m"). Let's
give "--list" the same treatment.

This also makes it consistent with the similar "git tag -l"
option.

We didn't do this originally because "--create-reflog" was
squatting on the "-l" option. Now that we've deprecated that
use for long enough, we can make the switch.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>