stash: fix "--staged" with binary files

"git stash --staged" errors out when given binary files, after saving the
stash.

This behaviour dates back to the addition of the feature in 41a28eb
(stash: implement '--staged' option for 'push' and 'save', 2021-10-18).
Adding the "--binary" option of "diff-tree" fixes this. The "diff-tree" call
in stash_patch() also omits "--binary", but that is fine since binary files
cannot be selected interactively.

Helped-By: Jeff King <peff@peff.net>
Helped-By: Randall S. Becker <randall.becker@nexbridge.ca>
Signed-off-by: Adam Johnson <me@adamj.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: adamchainz

builtin/stash.c

@@ -1147,8 +1147,8 @@ static int stash_staged(struct stash_info *info, struct strbuf *out_patch,
 	}
 
 	cp_diff_tree.git_cmd = 1;
-	strvec_pushl(&cp_diff_tree.args, "diff-tree", "-p", "-U1", "HEAD",
-		     oid_to_hex(&info->w_tree), "--", NULL);
+	strvec_pushl(&cp_diff_tree.args, "diff-tree", "-p", "--binary",
+		     "-U1", "HEAD", oid_to_hex(&info->w_tree), "--", NULL);
 	if (pipe_command(&cp_diff_tree, NULL, 0, out_patch, 0, NULL, 0)) {
 		ret = -1;
 		goto done;

t/t3903-stash.sh

@@ -318,6 +318,15 @@ test_expect_success 'stash --staged' '
 	test bar,bar4 = $(cat file),$(cat file2)
 '
 
+test_expect_success 'stash --staged with binary file' '
+	printf "\0" >file &&
+	git add file &&
+	git stash --staged &&
+	git stash pop &&
+	printf "\0" >expect &&
+	test_cmp expect file
+'
+
 test_expect_success 'dont assume push with non-option args' '
 	test_must_fail git stash -q drop 2>err &&
 	test_i18ngrep -e "subcommand wasn'\''t specified; '\''push'\'' can'\''t be assumed due to unexpected token '\''drop'\''" err