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>

t/README

@@ -340,7 +340,11 @@ Don't:
  - use perl without spelling it as "$PERL_PATH". This is to help our
    friends on Windows where the platform Perl often adds CR before
    the end of line, and they bundle Git with a version of Perl that
-   does not do so, whose path is specified with $PERL_PATH.
+   does not do so, whose path is specified with $PERL_PATH. Note that we
+   provide a "perl" function which uses $PERL_PATH under the hood, so
+   you do not need to worry when simply running perl in the test scripts
+   (but you do, for example, on a shebang line or in a sub script
+   created via "write_script").
 
  - use sh without spelling it as "$SHELL_PATH", when the script can
    be misinterpreted by broken platform shell (e.g. Solaris).
@@ -387,7 +391,7 @@ of the test_* functions (see the "Test harness library" section
 below), e.g.:
 
     test_expect_success PERL 'I need Perl' '
-        "$PERL_PATH" -e "hlagh() if unf_unf()"
+        perl -e "hlagh() if unf_unf()"
     '
 
 The advantage of skipping tests like this is that platforms that don't
@@ -520,7 +524,7 @@ library for your script to use.
 
 	test_external \
 	    'GitwebCache::*FileCache*' \
-	    "$PERL_PATH" "$TEST_DIRECTORY"/t9503/test_cache_interface.pl
+	    perl "$TEST_DIRECTORY"/t9503/test_cache_interface.pl
 
    If the test is outputting its own TAP you should set the
    test_external_has_tap variable somewhere before calling the first
@@ -536,7 +540,7 @@ library for your script to use.
 
 	test_external_without_stderr \
 	    'Perl API' \
-	    "$PERL_PATH" "$TEST_DIRECTORY"/t9700/test.pl
+	    perl "$TEST_DIRECTORY"/t9700/test.pl
 
  - test_expect_code <exit-code> <command>
 

t/test-lib-functions.sh

@@ -710,3 +710,7 @@ test_ln_s_add () {
 		git update-index --add --cacheinfo 120000 $ln_s_obj "$2"
 	fi
 }
+
+perl () {
+	command "$PERL_PATH" "$@"
+}