revisions API users: add "goto cleanup" for release_revisions()

Add a release_revisions() to various users of "struct rev_info" which
requires a minor refactoring to a "goto cleanup" pattern to use that
function.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff-files.c

@@ -77,8 +77,12 @@ int cmd_diff_files(int argc, const char **argv, const char *prefix)
 
 	if (read_cache_preload(&rev.diffopt.pathspec) < 0) {
 		perror("read_cache_preload");
-		return -1;
+		result = -1;
+		goto cleanup;
 	}
+cleanup:
 	result = run_diff_files(&rev, options);
-	return diff_result_code(&rev.diffopt, result);
+	result = diff_result_code(&rev.diffopt, result);
+	release_revisions(&rev);
+	return result;
 }