Scriptlets used form inside this test began with hardcoded "#!/bin/sh".
By setting SHELL_PATH the user is already telling us that what the vendor
has in /bin/sh isn't POSIX enough, and we really should try to honor that
request.

Originally noticed by SungHyun Nam who later tested this patch and
verified that it fixes the issue on Solaris 9.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>