Git.pm: add the "use warnings" pragma

Git.pm: add the "use warnings" pragma

Amend Git.pm to load the "warnings" pragma like the rest of the code
in perl/ in addition to the existing "strict" pragma. This is
considered the bare minimum best practice in Perl.

Ever since this code was introduced in b1edc53d06 ("Introduce
Git.pm (v4)", 2006-06-24) it's only been using "strict", not
"warnings".

This leaves contrib/buildsystems/Generators/{QMake,VCproj}.pm and
contrib/mw-to-git/Git/Mediawiki.pm without "use warnings". Amending
those would be a sensible follow-up change, but I don't have an easy
way to test those so I'm not changing them.

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