stash: fix handling removed files with --keep-index

git stash push --keep-index is supposed to keep all changes that have
been added to the index, both in the index and on disk.

Currently this doesn't behave correctly when a file is removed from
the index.  Instead of keeping it deleted on disk, --keep-index
currently restores the file.

Fix that behaviour by using 'git checkout' in no-overlay mode which
can faithfully restore the index and working tree.  This also
simplifies the code.

Note that this will overwrite untracked files if the untracked file
has the same name as a file that has been deleted in the index.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/stash.c

@@ -1390,30 +1390,16 @@ static int do_push_stash(const struct pathspec *ps, const char *stash_msg, int q
 		}
 
 		if (keep_index == 1 && !is_null_oid(&info.i_tree)) {
-			struct child_process cp_ls = CHILD_PROCESS_INIT;
-			struct child_process cp_checkout = CHILD_PROCESS_INIT;
-			struct strbuf out = STRBUF_INIT;
-
-			if (reset_tree(&info.i_tree, 0, 1)) {
-				ret = -1;
-				goto done;
-			}
-
-			cp_ls.git_cmd = 1;
-			argv_array_pushl(&cp_ls.args, "ls-files", "-z",
-					 "--modified", "--", NULL);
-
-			add_pathspecs(&cp_ls.args, ps);
-			if (pipe_command(&cp_ls, NULL, 0, &out, 0, NULL, 0)) {
-				ret = -1;
-				goto done;
-			}
+			struct child_process cp = CHILD_PROCESS_INIT;
 
-			cp_checkout.git_cmd = 1;
-			argv_array_pushl(&cp_checkout.args, "checkout-index",
-					 "-z", "--force", "--stdin", NULL);
-			if (pipe_command(&cp_checkout, out.buf, out.len, NULL,
-					 0, NULL, 0)) {
+			cp.git_cmd = 1;
+			argv_array_pushl(&cp.args, "checkout", "--no-overlay",
+					 oid_to_hex(&info.i_tree), "--", NULL);
+			if (!ps->nr)
+				argv_array_push(&cp.args, ":/");
+			else
+				add_pathspecs(&cp.args, ps);
+			if (run_command(&cp)) {
 				ret = -1;
 				goto done;
 			}

t/t3903-stash.sh

@@ -1216,4 +1216,11 @@ test_expect_success 'stash works when user.name and user.email are not set' '
 	)
 '
 
+test_expect_success 'stash --keep-index with file deleted in index does not resurrect it on disk' '
+	test_commit to-remove to-remove &&
+	git rm to-remove &&
+	git stash --keep-index &&
+	test_path_is_missing to-remove
+'
+
 test_done