commit: add repository argument to set_commit_buffer

Add a repository argument to allow callers of set_commit_buffer to
be more specific about which repository to handle. This is a small
mechanical change; it doesn't change the implementation to handle
repositories other than the_repository yet.

As with the previous commits, use a macro to catch callers passing a
repository other than the_repository at compile time.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -95,7 +95,8 @@ void parse_commit_or_die(struct commit *item);
  * Associate an object buffer with the commit. The ownership of the
  * memory is handed over to the commit, and must be free()-able.
  */
-void set_commit_buffer(struct commit *, void *buffer, unsigned long size);
+#define set_commit_buffer(r, c, b, s) set_commit_buffer_##r(c, b, s)
+void set_commit_buffer_the_repository(struct commit *, void *buffer, unsigned long size);
 
 /*
  * Get any cached object buffer associated with the commit. Returns NULL