Upspin Git repositories on github.com

[Andrew Gerrand]

[FYI only; no action is required]

The canonical host for the Upspin Git repositories is now github.com/upspin, and the project now prefers GitHub Pull Requests for accepting source contributions.

The Upspin Git repositories and Gerrit instances hosted at upspin.googlesource.com have been archived, and are no longer publicly available.

This should have no impact on Upspin users, but does change things a bit for contributors. The contribution guidelines have been updated to reflect this.

Please let me know if you notice any problems.

Cheers,
Andrew