t9164: use test_must_fail only on git commands

The `test_must_fail` function should only be used for git commands;
we are not in the business of catching segmentation fault by external
commands.  Shell helper functions test_cmp and svn_cmd used in this
script are wrappers around external commands, so just use `! cmd`
instead of `test_must_fail cmd`

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t9164-git-svn-dcommit-concurrent.sh b/t/t9164-git-svn-dcommit-concurrent.sh
index 90346ff..8466269 100755
--- a/t/t9164-git-svn-dcommit-concurrent.sh
+++ b/t/t9164-git-svn-dcommit-concurrent.sh
@@ -92,7 +92,7 @@
 		echo 1 >> file &&
 		svn_cmd commit -m "changing file" &&
 		svn_cmd up &&
-		test_must_fail test_cmp auto_updated_file au_file_saved
+		! test_cmp auto_updated_file au_file_saved
 	)
 '
 
@@ -103,7 +103,7 @@
 		echo 2 >> file &&
 		svn_cmd commit -m "changing file once again" &&
 		echo 3 >> file &&
-		test_must_fail svn_cmd commit -m "this commit should fail" &&
+		! svn_cmd commit -m "this commit should fail" &&
 		svn_cmd revert file
 	)
 '