commit: add repository argument to lookup_commit

Add a repository argument to allow callers of lookup_commit 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

@@ -63,7 +63,8 @@ enum decoration_type {
 void add_name_decoration(enum decoration_type type, const char *name, struct object *obj);
 const struct name_decoration *get_name_decoration(const struct object *obj);
 
-struct commit *lookup_commit(const struct object_id *oid);
+#define lookup_commit(r, o) lookup_commit_##r(o)
+struct commit *lookup_commit_the_repository(const struct object_id *oid);
 #define lookup_commit_reference(r, o) \
 		lookup_commit_reference_##r(o)
 struct commit *lookup_commit_reference_the_repository(const struct object_id *oid);