Make git add -n and git -u -n output consistent

Make git add -n and git -u -n output consistent

Output format from "git add -n $path" lists path to blobs that are going
to be added on a single line, separated with SP.  On the other hand, the
suggested "git add -u -n" shows one path per line, like "add '<file>'\n".
Of course, these two are inconsistent.

Plain "git add -n" can afford to only say names of paths, as all it does
is to add (update).  However, "git add -u" needs to be able to express
"remove" somehow.  So if we need to have them formatted the same way, we
need to unify with the "git add -n -u" format.  Incidentally, this is
consistent with how 'update-index' says it.

This changes the output from "git add -n $paths" but as a general
principle, output from Porcelain commands is a fair game for improvements
and not for script consumption.

Signed-off-by: Gustaf Hendeby <hendeby@isy.liu.se>
Signed-off-by: Junio C Hamano <gitster@pobox.com>