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.
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);
