t9100: fix breakage when SHELL_PATH is not /bin/sh

bcb11f1 (mingw: mark t9100's test cases with appropriate prereqs, 2016-01-27)
replaced "/bin/sh" in exec.sh by the shell specified in SHELL_PATH, but
that breaks the subtest which checks for a specific checksum of a tree
containing.

Revert that change that was not explained in the commit message anyways
(exec.sh is never executed).

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9100-git-svn-basic.sh

@@ -30,7 +30,8 @@ test_expect_success \
 		echo "deep dir" >dir/a/b/c/d/e/file &&
 		mkdir bar &&
 		echo "zzz" >bar/zzz &&
-		write_script exec.sh </dev/null &&
+		echo "#!/bin/sh" >exec.sh &&
+		chmod +x exec.sh &&
 		svn_cmd import -m "import for git svn" . "$svnrepo" >/dev/null
 	) &&
 	rm -rf import &&