690 no metadata rcheckin

Initial attempt at fixing #690. The rcheckin command does not inherit from Checkin and thus doesn't appear to obey the config field `git-tfs.export-metadatas`.

My approach is twofold:

1. extract from `Fetch.cs` the code which sets up repositories and remotes for metadata
2. Call this logic from `Rcheckin.cs` as well as `Fetch.cs`

I wasn't sure how to really get this under test.

Please let me know if my contribution can be improved in anyway.