commit: add repository argument to lookup_commit_reference

Add a repository argument to allow callers of lookup_commit_reference
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>
diff --git a/merge-recursive.c b/merge-recursive.c
index fbb044e..41366e7 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -1192,9 +1192,9 @@ static int merge_submodule(struct merge_options *o,
 		return 0;
 	}
 
-	if (!(commit_base = lookup_commit_reference(base)) ||
-	    !(commit_a = lookup_commit_reference(a)) ||
-	    !(commit_b = lookup_commit_reference(b))) {
+	if (!(commit_base = lookup_commit_reference(the_repository, base)) ||
+	    !(commit_a = lookup_commit_reference(the_repository, a)) ||
+	    !(commit_b = lookup_commit_reference(the_repository, b))) {
 		output(o, 1, _("Failed to merge submodule %s (commits not present)"), path);
 		return 0;
 	}