t0021: avoid grepping for a Perl-specific string at filter output

This test sets the t0021/rot13-filter.pl script as a long-running process filter for a git checkout command. It then expects the filter to fail producing a specific error message at stderr. In the following commits we are going to replace the script with a C test-tool helper, but the test currently expects the error message in a Perl-specific format. That is, when you call `die <msg>` in Perl, it emits "<msg> at - line 1." In preparation for the conversion, let's avoid the Perl-specific part and only grep for <msg> itself. Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br> Signed-off-by: Junio C Hamano <gitster@pobox.com>