stash: drop dirty worktree check on apply

stash: drop dirty worktree check on apply

Before we apply a stash, we make sure there are no changes
in the worktree that are not in the index. This check dates
back to the original git-stash.sh, and is presumably
intended to prevent changes in the working tree from being
accidentally lost during the merge.

However, this check has two problems:

  1. It is overly restrictive. If my stash changes only file
     "foo", but "bar" is dirty in the working tree, it will
     prevent us from applying the stash.

  2. It is redundant. We don't touch the working tree at all
     until we actually call merge-recursive. But it has its
     own (much more accurate) checks to avoid losing working
     tree data, and will abort the merge with a nicer
     message telling us which paths were problems.

So we can simply drop the check entirely.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>