git-svn: avoid warning on undef readline()

git-svn: avoid warning on undef readline()

Change code in Git.pm that sometimes calls chomp() on undef to only do
so the value is defined.

This code has been chomping undef values ever since it was added in
b26098fc2f ("git-svn: reduce scope of input record separator change",
2016-10-14), but started warning due to the introduction of "use
warnings" to Git.pm in my f0e19cb7ce ("Git.pm: add the "use warnings"
pragma", 2018-02-25) released with 2.17.0.

Since this function will return undef in those cases it's still
possible that the code using it will warn if it does a chomp of its
own, as the code added in b26098fc2f ("git-svn: reduce scope of input
record separator change", 2016-10-14) might do, but since git-svn has
"use warnings" already that's clearly not a codepath that's going to
warn.

See https://public-inbox.org/git/86h8oobl36.fsf@phe.ftfl.ca/ for the
original report.

Reported-by: Joseph Mingrone <jrm@ftfl.ca>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Improved-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>