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/t2000-checkout-cache-clash.sh b/t/t2000-checkout-cache-clash.sh
index 5141fab..f7e1a73 100755
--- a/t/t2000-checkout-cache-clash.sh
+++ b/t/t2000-checkout-cache-clash.sh
@@ -38,7 +38,7 @@
 
 test_expect_success \
     'git checkout-index without -f should fail on conflicting work tree.' \
-    '! git checkout-index -a'
+    'test_must_fail git checkout-index -a'
 
 test_expect_success \
     'git checkout-index with -f should succeed.' \