[manjari25]

Why?

The tango analyzer needs the commit a new batch is analyzed against, and nothing supplies it. The queue has no branch of its own, and deployments run git in different places.

What?

• Adds baserevision.FromGit(factory, targets), which returns the function Params.BaseRevision already takes. It works over any gitworkspace.Factory, so where git runs is the deployment's choice.
• Each call fetches the queue's target branch and returns FETCH_HEAD as a full commit SHA. FETCH_HEAD is used rather than <remote>/<branch>, because a clone whose fetch refspec doesn't cover the branch would leave the tracking ref stale.
• Fails closed: an unknown queue, a failed fetch, or output that isn't a full SHA is an error.
• Calls sharing a repo are serialized, since concurrent fetches into one clone can fail on a ref lock and overwrite each other's FETCH_HEAD.

Test Plan

• Unit tests added
• CI

Issue