userdiff/perl: match full line of POD headers

userdiff/perl: match full line of POD headers

The builtin perl userdiff driver is not greedy enough about catching
POD header lines.  Capture the whole line, so instead of just
declaring that we are in some "@@ =head1" section, diff/grep output
can explain that the enclosing section is about "@@ =head1 OPTIONS".

Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>