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>

t/t7003-filter-branch.sh

@@ -147,7 +147,8 @@ test_expect_success 'use index-filter to move into a subdirectory' '
 	          GIT_INDEX_FILE=\$GIT_INDEX_FILE.new \
 			git update-index --index-info &&
 		  mv \"\$GIT_INDEX_FILE.new\" \"\$GIT_INDEX_FILE\"" directorymoved &&
-	test -z "$(git diff HEAD directorymoved:newsubdir)"'
+	git diff --exit-code HEAD directorymoved:newsubdir
+'
 
 test_expect_success 'stops when msg filter fails' '
 	old=$(git rev-parse HEAD) &&