t5150: skip request-pull test if Perl is disabled

t5150: skip request-pull test if Perl is disabled

The git-request-pull.sh script invokes Perl, so it requires Perl to be
available, but the associated test t5150 does not skip itself when Perl
has been disabled, which then makes subtest 4 through 10 fail. Subtest 3
still passes, but for the wrong reasons (it expects git-request-pull to
fail, and it does fail when Perl is not available). The initial two
subtests that do pass are only doing setup.

To prevent t5150 from failing the build when NO_PERL=1, add a check that
sets skip_all when "! test_have_prereq PERL", just like how for example
t3701-add-interactive skips itself when Perl has been disabled.

Signed-off-by: Ruud van Asseldonk <dev@veniogames.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>