git: fix perl binding installation

Git.pm wasn't ending up in the package because do_install removed it
with a misleading comment about multilib. Fix the problem at the source
so that Git.pm ends up in the correct dir *and* doesn't get deleted.

The perl.mak file gets updated during the regular do_install, so add an
extra make install step to install the *.pm files to the correct
directory.

This is the first step in making git-perltools actually work.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>