fx revert

//tools/revert (fx revert) is a host tool for orchestrating pure reverts of submitted Gerrit changes across Fuchsia repositories (e.g. fuchsia.git and vendor/google).

Reverts are created server-side through Gerrit's REST API rather than by pushing locally generated commits. This produces authentic Gerrit reverts that Rubber Stamper (rubber-stamper@appspot.gserviceaccount.com) recognizes and approves automatically, so the tree can be unblocked without waiting for a human code review.

The tool is under active development; usage documentation will be added as functionality lands.