unpack-trees: mark new entries skip-worktree appropriately

Sparse checkout narrows worktree down based on the skip-worktree bit
before and after $GIT_DIR/info/sparse-checkout application. If it does
not have that bit before but does after, a narrow is detected and the
file will be removed from worktree.

New files added by merge, however, does not have skip-worktree bit. If
those files appear to be outside checkout area, the same rule applies:
the file gets removed from worktree even though they don't exist in
worktree.

Just pretend they have skip-worktree before in that case, so the rule
is ignored.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1011-read-tree-sparse-checkout.sh

@@ -146,7 +146,7 @@ test_expect_success 'read-tree adds to worktree, absent case' '
 	test ! -f sub/added
 '
 
-test_expect_failure 'read-tree adds to worktree, dirty case' '
+test_expect_success 'read-tree adds to worktree, dirty case' '
 	echo init.t >.git/info/sparse-checkout &&
 	git checkout -f removed &&
 	mkdir sub &&

unpack-trees.c

@@ -1096,6 +1096,8 @@ static int merged_entry(struct cache_entry *merge, struct cache_entry *old,
 	if (!old) {
 		if (verify_absent(merge, "overwritten", o))
 			return -1;
+		if (!o->skip_sparse_checkout && will_have_skip_worktree(merge, o))
+			update |= CE_SKIP_WORKTREE;
 		invalidate_ce_path(merge, o);
 	} else if (!(old->ce_flags & CE_CONFLICTED)) {
 		/*