am: add --show-current-patch

am: add --show-current-patch

Pointing the user to $GIT_DIR/rebase-apply may encourage them to mess
around in there, which is not a good thing. With this, the user does
not have to keep the path around somewhere (because after a couple of
commands, the path may be out of scrollback buffer) when they need to
look at the patch.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>