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/t4200-rerere.sh

@@ -41,7 +41,7 @@ git commit -q -a -m second
 
 test_expect_success 'nothing recorded without rerere' '
 	(rm -rf .git/rr-cache; git config rerere.enabled false) &&
-	! git merge first &&
+	test_must_fail git merge first &&
 	! test -d .git/rr-cache
 '
 
@@ -50,7 +50,7 @@ test_expect_success 'conflicting merge' '
 	git reset --hard &&
 	mkdir .git/rr-cache &&
 	git config --unset rerere.enabled &&
-	! git merge first
+	test_must_fail git merge first
 '
 
 sha1=$(sed -e 's/	.*//' .git/rr-cache/MERGE_RR)
@@ -61,7 +61,7 @@ test_expect_success 'rerere.enabled works, too' '
 	rm -rf .git/rr-cache &&
 	git config rerere.enabled true &&
 	git reset --hard &&
-	! git merge first &&
+	test_must_fail git merge first &&
 	grep ======= $rr/preimage
 '
 
@@ -120,7 +120,7 @@ test_expect_success 'another conflicting merge' '
 	git checkout -b third master &&
 	git show second^:a1 | sed "s/To die: t/To die! T/" > a1 &&
 	git commit -q -a -m third &&
-	! git pull . first
+	test_must_fail git pull . first
 '
 
 git show first:a1 | sed 's/To die: t/To die! T/' > expect
@@ -175,7 +175,7 @@ test_expect_success 'file2 added differently in two branches' '
 	echo Bello > file2 &&
 	git add file2 &&
 	git commit -m version2 &&
-	! git merge fourth &&
+	test_must_fail git merge fourth &&
 	sha1=$(sed -e "s/	.*//" .git/rr-cache/MERGE_RR) &&
 	rr=.git/rr-cache/$sha1 &&
 	echo Cello > file2 &&