diff --git a/commit.h b/commit.h
index ce0b436..620e936 100644
--- a/commit.h
+++ b/commit.h
@@ -19,6 +19,7 @@
 extern const char *commit_type;
 
 struct commit *lookup_commit(unsigned char *sha1);
+struct commit *lookup_commit_reference(unsigned char *sha1);
 
 int parse_commit_buffer(struct commit *item, void *buffer, unsigned long size);