transformChanges and workspaces don't really work together

Created by: LawnGnome

This is from https://github.com/sourcegraph/customer/issues/599, but was a known issue anyway: workspaces that would result in the same repo getting multiple branches with names set by transformChanges are currently unsupported because the transformation occurs after the changeset specs are built and validated.

Given #28246, I'm unsure if we actually want to fix this — it really depends on whether we intend to support transformChanges in the longer term as a non-experimental feature.