t7063: fix flaky untracked-cache test
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 <a13b1135e6f2f8530e85bc2896cd5ed89a6ed863@twopensource.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git