commit: save the commit buffer off when parsing a commit

object.

A fair number of the users potentially want to look at the
commit objects more closely, and if you worry about memory
leaking in certain applications, you can always do a

	free(commit->buffer);
	commit->buffer = NULL;

by hand after parsing them.

commit.h

@@ -14,6 +14,7 @@ struct commit {
 	unsigned long date;
 	struct commit_list *parents;
 	struct tree *tree;
+	const char *buffer;
 };
 
 extern const char *commit_type;