git-gui: Move load_config procedure below git-version selection

git-gui: Move load_config procedure below git-version selection

To better handle configuration options that contain LFs in their
values we want to use the new -z option available in git-config
version 1.5.3 and later.  To configure load_config based upon the
git version we need to move thos below the git-version computation.

No logic changes yet, just a minor reordering of the code.

Signed-off-by: Shawn O. Pearce <redacted>