Convert "! git" to "test_must_fail git"

test_must_fail will account for segfaults in git, so it should be used
instead of "! git"

This patch does not change any of the commands that use pipes.

Signed-off-by: Jared Hance <jaredhance@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9139-git-svn-non-utf8-commitencoding.sh

@@ -39,7 +39,7 @@ do
 	(
 		cd $H &&
 		git config --unset i18n.commitencoding &&
-		! git svn dcommit
+		test_must_fail git svn dcommit
 	)
 	'
 done