Move Git::SVN::get_tz to Git::get_tz_offset

Move Git::SVN::get_tz to Git::get_tz_offset

This function has utility outside of the SVN module for any routine
that needs the equivalent of GNU strftime's %z formatting option.
Move it to the top-level Git.pm so that non-SVN modules don't need to
import the SVN module to use it.

The rename makes the purpose of the function clearer.

Signed-off-by: Ben Walton <bdwalton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>