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/t4113-apply-ending.sh b/t/t4113-apply-ending.sh
index d741039..66fa515 100755
--- a/t/t4113-apply-ending.sh
+++ b/t/t4113-apply-ending.sh
@@ -30,7 +30,7 @@
 # test
 
 test_expect_success 'apply at the end' \
-    '! git apply --index test-patch'
+    'test_must_fail git apply --index test-patch'
 
 cat >test-patch <<\EOF
 diff a/file b/file
@@ -48,6 +48,6 @@
 git update-index file
 
 test_expect_success 'apply at the beginning' \
-	'! git apply --index test-patch'
+	'test_must_fail git apply --index test-patch'
 
 test_done