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>

git-stash.sh

@@ -344,9 +344,7 @@ apply_stash () {
 
 	assert_stash_like "$@"
 
-	git update-index -q --refresh &&
-	git diff-files --quiet --ignore-submodules ||
-		die 'Cannot apply to a dirty working tree, please stage your changes'
+	git update-index -q --refresh || die 'unable to refresh index'
 
 	# current index state
 	c_tree=$(git write-tree) ||

t/t3903-stash.sh

@@ -37,14 +37,26 @@ test_expect_success 'parents of stash' '
 	test_cmp output expect
 '
 
-test_expect_success 'apply needs clean working directory' '
-	echo 4 > other-file &&
+test_expect_success 'apply does not need clean working directory' '
+	echo 4 >other-file &&
 	git add other-file &&
-	echo 5 > other-file &&
-	test_must_fail git stash apply
+	echo 5 >other-file &&
+	git stash apply &&
+	echo 3 >expect &&
+	test_cmp expect file
+'
+
+test_expect_success 'apply does not clobber working directory changes' '
+	git reset --hard &&
+	echo 4 >file &&
+	test_must_fail git stash apply &&
+	echo 4 >expect &&
+	test_cmp expect file
 '
 
 test_expect_success 'apply stashed changes' '
+	git reset --hard &&
+	echo 5 >other-file &&
 	git add other-file &&
 	test_tick &&
 	git commit -m other-file &&