repo: move parser init out of module scope

We import the wrapper on the fly, so minimize how much code we run
in module scope.  It's pointless/wasted when importing.

Change-Id: I4a71c2030325d0a639585671cd7ebe8f22687ecd
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254072
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>