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>

t/t7063-status-untracked-cache.sh

@@ -412,7 +412,9 @@ test_expect_success 'create/modify files, some of which are gitignored' '
 	echo two bis >done/two &&
 	echo three >done/three && # three is gitignored
 	echo four >done/four && # four is gitignored at a higher level
-	echo five >done/five # five is not gitignored
+	echo five >done/five && # five is not gitignored
+	echo test >base && #we need to ensure that the root dir is touched
+	rm base
 '
 
 test_expect_success 'test sparse status with untracked cache' '