BaseTools: add centralized location for git config files

Before adding the git environment initialization script, add the
following files that will be pointed to after running said script:

- BaseTools/Conf/diff.order
- BaseTools/Conf/gitattributes

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>