t/: Use "test_must_fail git" instead of "! git"

This patch changes every occurrence of "! git" -- with the meaning
that a git call has to gracefully fail -- into "test_must_fail git".

This is useful to

 - make sure the test does not fail because of a signal,
   e.g. SIGSEGV, and

 - advertise the use of "test_must_fail" for new tests.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t5404-tracking-branches.sh b/t/t5404-tracking-branches.sh
index 64fe261..c240035 100755
--- a/t/t5404-tracking-branches.sh
+++ b/t/t5404-tracking-branches.sh
@@ -35,7 +35,9 @@
 	git commit -a -m aa-master
 '
 
-test_expect_success 'mixed-success push returns error' '! git push'
+test_expect_success 'mixed-success push returns error' '
+	test_must_fail git push
+'
 
 test_expect_success 'check tracking branches updated correctly after push' '
 	test "$(git rev-parse origin/master)" = "$(git rev-parse master)"