Fix the format script so that it works with deleted files. (flutter#20679)

When I rewrote the format script, I forgot to filter out deleted files from the list of files that it looks at. This filters the file list to exclude deleted files

Author: gspencergoog

ci/bin/format.dart

@@ -258,6 +258,7 @@ abstract class FormatChecker {
         'diff',
         '-U0',
         '--no-color',
+        '--diff-filter=d',
         '--name-only',
         baseGitRef,
         '--',