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>

t/t9106-git-svn-dcommit-clobber-series.sh

@@ -57,7 +57,7 @@ test_expect_success 'change file but in unrelated area' "
 test_expect_success 'attempt to dcommit with a dirty index' '
 	echo foo >>file &&
 	git add file &&
-	! git svn dcommit
+	test_must_fail git svn dcommit
 '
 
 test_done