Add a BEGIN block so that the gitrepo and checkout variables actually work

Add a BEGIN block so that the gitrepo and checkout variables actually work

The trick here is that the hash containing configuration settings is
being passed to a perl module's import method with a use statement.
That use statement includes an implicit BEGIN statement which makes
the import function get called at compile-time. So our settings for
gitrepo and checkout don't take effect unless we also put them in
their own BEGIN block.