t7063: fix flaky untracked-cache test

Dirty the test worktree's root directory, as the test expects. When testing the untracked-cache, we previously assumed that checking out master would be sufficient to mark the mtime of the worktree's root directory as racily-dirty. But sometimes, the checkout would happen at 12345.999 seconds and the status at 12346.001 seconds, meaning that the worktree's root directory would not be racily-dirty. And since it was not truly dirty, occasionally the test would fail. By making the root truly dirty, the test will always succeed. Tested by running a few hundred times. Signed-off-by: David Turner <dturner@twopensource.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>