t: use perl instead of "$PERL_PATH" where applicable

As of the last commit, we can use "perl" instead of
"$PERL_PATH" when running tests, as the former is now a
function which uses the latter. As the shorter "perl" is
easier on the eyes, let's switch to using it everywhere.

This is not quite a mechanical s/$PERL_PATH/perl/
replacement, though. There are some places where we invoke
perl from a script we generate on the fly, and those scripts
do not have access to our internal shell functions. The
result can be double-checked by running:

  ln -s /bin/false bin-wrappers/perl
  make test

which continues to pass even after this patch.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4020-diff-external.sh

@@ -177,7 +177,7 @@ test_expect_success 'no diff with -diff' '
 	git diff | grep Binary
 '
 
-echo NULZbetweenZwords | "$PERL_PATH" -pe 'y/Z/\000/' > file
+echo NULZbetweenZwords | perl -pe 'y/Z/\000/' > file
 
 test_expect_success 'force diff with "diff"' '
 	echo >.gitattributes "file diff" &&