Add an OCaml manual mirror to the git-svn scripts.
Merge pull request #3 from yallop/ocaml-manual-mirror

Add a mirror for the OCaml manual repository to the git-svn scripts.
Use 'checkout' instead of 'track' to follow upstream branches.

Tracking a branch created by git-svn no longer works as of git
1.8.3.2, since there's no valid upstream remote.