tg2quilt.mk: don't call git-status with git-commit options

git-status older than v1.7.0-rc0~137^2~17 used to take the same options
as git-commit.  So reimplement the needed logic using plumbing.  An
alternative would be using git commit --dry-run.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Tested-by: James Vega <jamessan@debian.org>
Closes: http://bugs.debian.org/581582