commit: add "lookup_commit_reference()" helper function

It's pretty much the same as "lookup_commit()", but it will take
tags too, and look up the commit (if any) associated with them.

commit.h

@@ -19,6 +19,7 @@ struct commit {
 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);