Perf: Don't perform merge operations for trivial merges.

When one side of a merge is treesame to the ancestor, we can take the other side and skip all the expensive merge operations. This optimization can only be performed when the generation of  REUC extension data is skipped.

src/merge.c

@@ -2018,6 +2018,26 @@ int git_merge_trees(
 	git_iterator_options iter_opts = GIT_ITERATOR_OPTIONS_INIT;
 	int error;
 
+	assert(out && repo);
+
+	/* if one side is treesame to the ancestor, take the other side */
+	if (ancestor_tree && merge_opts && (merge_opts->flags & GIT_MERGE_SKIP_REUC)) {
+		const git_tree *result = NULL;
+		const git_oid *ancestor_tree_id = git_tree_id(ancestor_tree);
+
+		if (our_tree && !git_oid_cmp(ancestor_tree_id, git_tree_id(our_tree)))
+			result = their_tree;
+		else if (their_tree && !git_oid_cmp(ancestor_tree_id, git_tree_id(their_tree)))
+			result = our_tree;
+
+		if (result) {
+			if ((error = git_index_new(out)) == 0)
+			error = git_index_read_tree(*out, result);
+
+			return error;
+		}
+	}
+
 	iter_opts.flags = GIT_ITERATOR_DONT_IGNORE_CASE;
 
 	if ((error = git_iterator_for_tree(