Fix 5501 test

Fix 5501 test

Not everybody can rely on /bin/sh to be sane, and we support
SHELL_PATH for that.  Use it.

mktemp(1) is not used anywhere else in the core git.  Do not
introduce dependency on it.

Not everybody's "which" gives a sane return value.  For example,
on Solaris 'which XXX' says "no XXX in /usr/bin /bin ..." and
exits with zero status.  The lesson here is to never use 'which'
in your scripts.

Signed-off-by: Junio C Hamano <junkio@twinsun.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>