stash: pass the pathspec argument to git reset

stash: pass the pathspec argument to git reset

For "git stash -p --no-keep-index", the pathspec argument is currently
not passed to "git reset".  This means that changes that are staged but
that are excluded from the pathspec still get unstaged by git stash -p.

Make sure that doesn't happen by passing the pathspec argument to the
git reset in question, bringing the behaviour in line with "git stash --
<pathspec>".

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>