commit-graph.c: make 'commit-graph-chain's read-only

In a previous commit, we made incremental graph layers read-only by
using 'git_mkstemp_mode' with permissions '0444'.

There is no reason that 'commit-graph-chain's should be modifiable by
the user, since they are generated at a temporary location and then
atomically renamed into place.

To ensure that these files are read-only, too, use
'hold_lock_file_for_update_mode' with the same read-only permission
bits, and let the umask and 'adjust_shared_perm' take care of the rest.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.c

@@ -1378,7 +1378,8 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 	if (ctx->split) {
 		char *lock_name = get_chain_filename(ctx->odb);
 
-		hold_lock_file_for_update(&lk, lock_name, LOCK_DIE_ON_ERROR);
+		hold_lock_file_for_update_mode(&lk, lock_name,
+					       LOCK_DIE_ON_ERROR, 0444);
 
 		fd = git_mkstemp_mode(ctx->graph_name, 0444);
 		if (fd < 0) {

t/t5324-split-commit-graph.sh

@@ -366,6 +366,8 @@ do
 		test_line_count = 1 graph-files &&
 		echo "$modebits" >expect &&
 		test_modebits $graphdir/graph-*.graph >actual &&
+		test_cmp expect actual &&
+		test_modebits $graphdir/commit-graph-chain >actual &&
 		test_cmp expect actual
 	'
 done <<\EOF