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/t9129-git-svn-i18n-commitencoding.sh

@@ -29,7 +29,7 @@ fi
 compare_svn_head_with () {
 	# extract just the log message and strip out committer info.
 	# don't use --limit here since svn 1.1.x doesn't have it,
-	LC_ALL="$a_utf8_locale" svn log `git svn info --url` | "$PERL_PATH" -w -e '
+	LC_ALL="$a_utf8_locale" svn log `git svn info --url` | perl -w -e '
 		use bytes;
 		$/ = ("-"x72) . "\n";
 		my @x = <STDIN>;