Let an author move their change request's head from the browser

Let an author move their change request's head from the browser

A request names the exact commit under review, so a fixup pushed after it
was opened is invisible until the head moves. The only mover was
`vole change update --head`, which writes the metadata branch — a write a
contributor cannot make. The person who owns the branch was the one person
who could not keep the request current.

The request page now offers the author (or a maintainer) a branch picker
that moves an open request's head to that branch's tip. It applies the
same rule the new-request form does: the head must be a branch tip, and a
contributor may only name a branch under their own namespace, which is
what the pre-receive hook would have let them push. Approvals stay in the
tree as history and stop matching, which is what a new head already means.

SetRequestHead now refuses a closed or merged request, so the CLI and the
browser share that rule instead of the web handler owning it alone.