t/README: test_must_fail is for testing Git

When a test wants to make sure there is no <string> in an output
file, we should just say "! grep string output".

"test_must_fail" is there only to test Git command and catch unusual
deaths we know about (e.g. segv) as an error, not as an expected
failure.  "test_must_fail grep string output" is unnecessary, as
we are not making sure the system binaries do not dump core or
anything like that.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/README

@@ -324,6 +324,9 @@ Don't:
    use 'test_must_fail git cmd'.  This will signal a failure if git
    dies in an unexpected way (e.g. segfault).
 
+   On the other hand, don't use test_must_fail for running regular
+   platform commands; just use '! cmd'.
+
  - 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