t/README: tests can use perl even with NO_PERL

The git build system supports a NO_PERL switch to avoid installing
perl bindings or other features (like "git add --patch") that rely on
perl on runtime, but even with NO_PERL it has not been possible for a
long time to run tests without perl.  Helpers such as

	nul_to_q () {
		"$PERL_PATH" -pe 'y/\000/Q/'
	}

use perl as a better tr or sed and are regularly used in tests without
worrying to add a PERL prerequisite.

Perl is portable enough that it seems fine to keep relying on it for
this kind of thing in tests (and more readable than the alternative of
trying to find POSIXy equivalents).  Update the test documentation to
clarify this.

Reported-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/README

@@ -629,11 +629,18 @@ See the prereq argument to the test_* functions in the "Test harness
 library" section above and the "test_have_prereq" function for how to
 use these, and "test_set_prereq" for how to define your own.
 
- - PERL & PYTHON
+ - PYTHON
 
-   Git wasn't compiled with NO_PERL=YesPlease or
-   NO_PYTHON=YesPlease. Wrap any tests that need Perl or Python in
-   these.
+   Git wasn't compiled with NO_PYTHON=YesPlease. Wrap any tests that
+   need Python with this.
+
+ - PERL
+
+   Git wasn't compiled with NO_PERL=YesPlease.
+
+   Even without the PERL prerequisite, tests can assume there is a
+   usable perl interpreter at $PERL_PATH, though it need not be
+   particularly modern.
 
  - POSIXPERM