git.el: find the git-status buffer whatever its name is

git-status used the buffer name to find git-status buffers, and that
can fail if the buffer has another name, for example when multiple
working directories is tracked.

Signed-off-by: Rémi Vanicat <vanicat@debian.org>
Acked-by: Alexandre Julliard <julliard@winehq.org>
Tested-by: Xavier Maillard <xma@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>