git-status: make porcelain more robust

git-status: make porcelain more robust

git status provides a porcelain mode for porcelain writers with a
supposedly stable (plumbing) interface.
7a76c28ff2 ("status: disable translation when --porcelain is used", 2014-03-20)
made sure that ahead/behind info is not translated (i.e. is stable).

Make sure that the remaining two strings (initial commit, detached head)
are stable, too.

These changes are for the v1 porcelain interface. While we do have a perfectly
stable v2 porcelain interface now, some tools (such as
powerline-gitstatus) are written against v1 and profit from fixing v1
without any changes on their side.

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