request-pull: stop depending on Perl

request-pull: stop depending on Perl

While git-request-pull(1) is written as a shell script, for it to
function we depend on Perl being available. The script gets installed
unconditionally though, regardless of whether or not Perl is even
available on the system. When it's not available, the `@PERL_PATH@`
variable may be substituted with a nonexistent executable path and thus
cause the script to fail.

Refactor the script so that it does not depend on Perl at all anymore.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>