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/test-lib-functions.sh

@@ -76,11 +76,11 @@ test_decode_color () {
 }
 
 nul_to_q () {
-	"$PERL_PATH" -pe 'y/\000/Q/'
+	perl -pe 'y/\000/Q/'
 }
 
 q_to_nul () {
-	"$PERL_PATH" -pe 'y/Q/\000/'
+	perl -pe 'y/Q/\000/'
 }
 
 q_to_cr () {
@@ -648,7 +648,7 @@ test_seq () {
 	2)	;;
 	*)	error "bug in the test script: not 1 or 2 parameters to test_seq" ;;
 	esac
-	"$PERL_PATH" -le 'print for $ARGV[0]..$ARGV[1]' -- "$@"
+	perl -le 'print for $ARGV[0]..$ARGV[1]' -- "$@"
 }
 
 # This function can be used to schedule some commands to be run