sequencer: handle merge_trees() errors correctly

The merge_trees() function returns 1 when the merge was clean, and
0 when merge conflicts were encountered. If even more serious errors
occur, though, it returns negative values.

This patch fixes sequencer's behavior in case of such serious errors.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

sequencer.c

@@ -410,6 +410,8 @@ static int do_recursive_merge(struct commit *base, struct commit *next,
 	clean = merge_trees(&o,
 			    head_tree,
 			    next_tree, base_tree, &result);
+	if (clean < 0)
+		return clean;
 
 	if (active_cache_changed &&
 	    write_locked_index(&the_index, &index_lock, COMMIT_LOCK))