t0021: use $PERL_PATH for rot13-filter.pl

The rot13-filter.pl script hardcodes "#!/usr/bin/perl", and
does not respect $PERL_PATH at all. That is a problem if the
system does not have perl at that path, or if it has a perl
that is too old to run a complicated script like the
rot13-filter (but PERL_PATH points to a more modern one).

We can fix this by using write_script() to create a new copy
of the script with the correct #!-line. In theory we could
move the whole script inside t0021-conversion.sh rather than
having it as an auxiliary file, but it's long enough that
it just makes things harder to read.

As a bonus, we can stop using the full path to the script in
the filter-process config we add (because the trash
directory is in our PATH). Not only is this shorter, but it
sidesteps any shell-quoting issues. The original was broken
when $TEST_DIRECTORY contained a space, because it was
interpolated in the outer script.

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

t/t0021-conversion.sh

@@ -13,6 +13,9 @@ tr \
   'nopqrstuvwxyzabcdefghijklmNOPQRSTUVWXYZABCDEFGHIJKLM'
 EOF
 
+write_script rot13-filter.pl "$PERL_PATH" \
+	<"$TEST_DIRECTORY"/t0021/rot13-filter.pl
+
 generate_random_characters () {
 	LEN=$1
 	NAME=$2
@@ -341,7 +344,7 @@ test_expect_success 'diff does not reuse worktree files that need cleaning' '
 '
 
 test_expect_success PERL 'required process filter should filter data' '
-	test_config_global filter.protocol.process "$TEST_DIRECTORY/t0021/rot13-filter.pl clean smudge" &&
+	test_config_global filter.protocol.process "rot13-filter.pl clean smudge" &&
 	test_config_global filter.protocol.required true &&
 	rm -rf repo &&
 	mkdir repo &&
@@ -434,7 +437,7 @@ test_expect_success PERL 'required process filter should filter data' '
 
 test_expect_success PERL 'required process filter takes precedence' '
 	test_config_global filter.protocol.clean false &&
-	test_config_global filter.protocol.process "$TEST_DIRECTORY/t0021/rot13-filter.pl clean" &&
+	test_config_global filter.protocol.process "rot13-filter.pl clean" &&
 	test_config_global filter.protocol.required true &&
 	rm -rf repo &&
 	mkdir repo &&
@@ -459,7 +462,7 @@ test_expect_success PERL 'required process filter takes precedence' '
 '
 
 test_expect_success PERL 'required process filter should be used only for "clean" operation only' '
-	test_config_global filter.protocol.process "$TEST_DIRECTORY/t0021/rot13-filter.pl clean" &&
+	test_config_global filter.protocol.process "rot13-filter.pl clean" &&
 	rm -rf repo &&
 	mkdir repo &&
 	(
@@ -494,7 +497,7 @@ test_expect_success PERL 'required process filter should be used only for "clean
 '
 
 test_expect_success PERL 'required process filter should process multiple packets' '
-	test_config_global filter.protocol.process "$TEST_DIRECTORY/t0021/rot13-filter.pl clean smudge" &&
+	test_config_global filter.protocol.process "rot13-filter.pl clean smudge" &&
 	test_config_global filter.protocol.required true &&
 
 	rm -rf repo &&
@@ -554,7 +557,7 @@ test_expect_success PERL 'required process filter should process multiple packet
 '
 
 test_expect_success PERL 'required process filter with clean error should fail' '
-	test_config_global filter.protocol.process "$TEST_DIRECTORY/t0021/rot13-filter.pl clean smudge" &&
+	test_config_global filter.protocol.process "rot13-filter.pl clean smudge" &&
 	test_config_global filter.protocol.required true &&
 	rm -rf repo &&
 	mkdir repo &&
@@ -573,7 +576,7 @@ test_expect_success PERL 'required process filter with clean error should fail'
 '
 
 test_expect_success PERL 'process filter should restart after unexpected write failure' '
-	test_config_global filter.protocol.process "$TEST_DIRECTORY/t0021/rot13-filter.pl clean smudge" &&
+	test_config_global filter.protocol.process "rot13-filter.pl clean smudge" &&
 	rm -rf repo &&
 	mkdir repo &&
 	(
@@ -624,7 +627,7 @@ test_expect_success PERL 'process filter should restart after unexpected write f
 '
 
 test_expect_success PERL 'process filter should not be restarted if it signals an error' '
-	test_config_global filter.protocol.process "$TEST_DIRECTORY/t0021/rot13-filter.pl clean smudge" &&
+	test_config_global filter.protocol.process "rot13-filter.pl clean smudge" &&
 	rm -rf repo &&
 	mkdir repo &&
 	(
@@ -663,7 +666,7 @@ test_expect_success PERL 'process filter should not be restarted if it signals a
 '
 
 test_expect_success PERL 'process filter abort stops processing of all further files' '
-	test_config_global filter.protocol.process "$TEST_DIRECTORY/t0021/rot13-filter.pl clean smudge" &&
+	test_config_global filter.protocol.process "rot13-filter.pl clean smudge" &&
 	rm -rf repo &&
 	mkdir repo &&
 	(

t/t0021/rot13-filter.pl

@@ -1,4 +1,3 @@
-#!/usr/bin/perl
 #
 # Example implementation for the Git filter protocol version 2
 # See Documentation/gitattributes.txt, section "Filter Protocol"