t1301: fix wrong template dir for git-init

The template dir prepared in test case "forced modes" is not used as
expected because a wrong template dir is provided to "git init". This is
because the $CWD for "git-init" command is a sibling directory alongside
the template directory. Change it to the right template directory and
add a protection test using "test_path_is_file".

The wrong template directory was introduced by mistake in commit
e1df7fe (init: make --template path relative to $CWD, 2019-05-10).

Signed-off-by: Jiang Xin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jiangxin

t/t1301-shared-repo.sh

@@ -140,7 +140,8 @@ test_expect_success POSIXPERM 'forced modes' '
 	(
 		cd new &&
 		umask 002 &&
-		git init --shared=0660 --template=templates &&
+		git init --shared=0660 --template=../templates &&
+		test_path_is_file .git/hooks/post-update &&
 		>frotz &&
 		git add frotz &&
 		git commit -a -m initial &&