builtin-merge: refresh the index before calling a strategy

In case a file is touched but has no real changes then we just have to
update the index and should not error out.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

builtin-merge.c

@@ -547,6 +547,16 @@ static int try_merge_strategy(const char *strategy, struct commit_list *common,
 	int i = 0, ret;
 	struct commit_list *j;
 	struct strbuf buf;
+	int index_fd;
+	struct lock_file *lock = xcalloc(1, sizeof(struct lock_file));
+
+	index_fd = hold_locked_index(lock, 1);
+	refresh_cache(REFRESH_QUIET);
+	if (active_cache_changed &&
+			(write_cache(index_fd, active_cache, active_nr) ||
+			 commit_locked_index(lock)))
+		return error("Unable to write index.");
+	rollback_lock_file(lock);
 
 	if (!strcmp(strategy, "recursive") || !strcmp(strategy, "subtree")) {
 		int clean;

t/t7600-merge.sh

@@ -511,4 +511,13 @@ test_expect_success 'in-index merge' '
 
 test_debug 'gitk --all'
 
+test_expect_success 'refresh the index before merging' '
+	git reset --hard c1 &&
+	sleep 1 &&
+	touch file &&
+	git merge c3
+'
+
+test_debug 'gitk --all'
+
 test_done