stash: keep untracked files intact in stash -k

stash: keep untracked files intact in stash -k

Currently when there are untracked changes in a file "one" and in a file
"two" in the repository and the user uses:

    git stash push -k one

all changes in "two" are wiped out completely.  That is clearly not the
intended result.  Make sure that only the files given in the pathspec
are changed when git stash push -k <pathspec> is used.

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>