read-tree: --trivial

This adds an option --trivial to restrict 3-way 'read-tree -m -u'
to happen only if there is no file-level merging required.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

read-tree.c

@@ -13,6 +13,8 @@
 static int merge = 0;
 static int update = 0;
 static int index_only = 0;
+static int nontrivial_merge = 0;
+static int trivial_merges_only = 0;
 
 static int head_idx = -1;
 static int merge_size = 0;
@@ -275,6 +277,9 @@ static int unpack_trees(merge_fn_t fn)
 	if (unpack_trees_rec(posns, len, "", fn, &indpos))
 		return -1;
 
+	if (trivial_merges_only && nontrivial_merge)
+		die("Merge requires file-level merging");
+
 	check_updates(active_cache, active_nr);
 	return 0;
 }
@@ -460,6 +465,8 @@ static int threeway_merge(struct cache_entry **stages)
 		verify_uptodate(index);
 	}
 
+	nontrivial_merge = 1;
+
 	/* #2, #3, #4, #6, #7, #9, #11. */
 	count = 0;
 	if (!head_match || !remote_match) {
@@ -629,6 +636,11 @@ int main(int argc, char **argv)
 			continue;
 		}
 
+		if (!strcmp(arg, "--trivial")) {
+			trivial_merges_only = 1;
+			continue;
+		}
+
 		/* "-m" stands for "merge", meaning we start in stage 1 */
 		if (!strcmp(arg, "-m")) {
 			if (stage || merge)