git-instaweb: use @SHELL_PATH@ instead of /bin/sh

git-instaweb: use @SHELL_PATH@ instead of /bin/sh

If the user has configured a value for SHELL_PATH then
be sure to use it for any generated scripts instead of
hard-coding /bin/sh.

The first line of the script is handled specially, but
the embedded #!/bin/sh line in the here document will
not be automatically updated unless it uses @SHELL_PATH@.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>