prepare_revision_walk(): check for return value in all places

Even the documentation tells us:

	You should check if it returns any error (non-zero return
	code) and if it does not, you can start using get_revision()
	to do the iteration.

In preparation for this commit, I grepped all occurrences of
prepare_revision_walk and added error messages, when there were none.

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

Author: stefanbeller

builtin/branch.c

@@ -653,7 +653,9 @@ static int print_ref_list(int kinds, int detached, int verbose, int abbrev, stru
 		add_pending_object(&ref_list.revs,
 				   (struct object *) filter, "");
 		ref_list.revs.limited = 1;
-		prepare_revision_walk(&ref_list.revs);
+
+		if (prepare_revision_walk(&ref_list.revs))
+			die(_("revision walk setup failed"));
 		if (verbose)
 			ref_list.maxwidth = calc_maxwidth(&ref_list);
 	}

builtin/commit.c

@@ -989,7 +989,8 @@ static const char *find_author_by_nickname(const char *name)
 	revs.mailmap = &mailmap;
 	read_mailmap(revs.mailmap, NULL);
 
-	prepare_revision_walk(&revs);
+	if (prepare_revision_walk(&revs))
+		die(_("revision walk setup failed"));
 	commit = get_revision(&revs);
 	if (commit) {
 		struct pretty_print_context ctx = {0};

remote.c

@@ -1898,7 +1898,8 @@ int stat_tracking_info(struct branch *branch, int *num_ours, int *num_theirs)
 
 	init_revisions(&revs, NULL);
 	setup_revisions(rev_argc, rev_argv, &revs, NULL);
-	prepare_revision_walk(&revs);
+	if (prepare_revision_walk(&revs))
+		die("revision walk setup failed");
 
 	/* ... and count the commits on each side. */
 	*num_ours = 0;