t7003: fix subdirectory-filter test
The test would not fail if the filtering failed to do anything, since
in
test -z "$(git diff HEAD directorymoved:newsubdir)"'
'directorymoved:newsubdir' is not valid, so git-diff fails without
printing anything on stdout. But then the exit status of git-diff is
lost, whereas test -z "" succeeds.
Use 'git diff --exit-code' instead, which does the right thing and has
the added bonus of showing the differences if there are any.
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>