submodule: use submodule repository when preparing summary

In show_submodule_header(), we gather the left and right commits
of the submodule repository, as well as the merge bases. However,
prepare_submodule_summary() initializes the rev_info with the_repository,
so we end up parsing the commit in the wrong repository.

This results in a fatal error in parse_commit_in_graph(), since the
passed item does not belong to the repository's commit graph.

Signed-off-by: Michael Forney <mforney@mforney.org>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.c

@@ -438,13 +438,13 @@ void handle_ignore_submodules_arg(struct diff_options *diffopt,
 	 */
 }
 
-static int prepare_submodule_summary(struct rev_info *rev, const char *path,
-		struct commit *left, struct commit *right,
+static int prepare_submodule_summary(struct repository *r, struct rev_info *rev,
+		const char *path, struct commit *left, struct commit *right,
 		struct commit_list *merge_bases)
 {
 	struct commit_list *list;
 
-	repo_init_revisions(the_repository, rev, NULL);
+	repo_init_revisions(r, rev, NULL);
 	setup_revisions(0, NULL, rev, NULL);
 	rev->left_right = 1;
 	rev->first_parent_only = 1;
@@ -632,7 +632,7 @@ void show_submodule_summary(struct diff_options *o, const char *path,
 		goto out;
 
 	/* Treat revision walker failure the same as missing commits */
-	if (prepare_submodule_summary(&rev, path, left, right, merge_bases)) {
+	if (prepare_submodule_summary(sub, &rev, path, left, right, merge_bases)) {
 		diff_emit_submodule_error(o, "(revision walker failed)\n");
 		goto out;
 	}