t: provide a perl() function which uses $PERL_PATH

Once upon a time, we assumed that calling a bare "perl" in the test scripts was OK, because we would find the perl from the user's PATH, and we were only asking that perl to do basic operations that work even on old versions of perl. Later, we found that some systems really prefer to use $PERL_PATH even for these basic cases, because the system perl misbehaves in some way (e.g., by handling line endings differently). We then switched "perl" invocations to "$PERL_PATH" to respect the user's choice. Having to use "$PERL_PATH" is ugly and cumbersome, though. Instead, let's provide a perl() shell function that tests can use, which will transparently do the right thing. Unfortunately, test writers still have to use $PERL_PATH in certain situations, so we still need to keep the advice in the README. Note that this may fix test failures in t5004, t5503, t6002, t6003, t6300, t8001, and t8002, depending on your system's perl setup. All of these can be detected by running: ln -s /bin/false bin-wrappers/perl make test which fails before this patch, and passes after. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>