rename GetMetadataBranchTree to GetMetadataRefTree

Take the ref name as a parameter rather than hardcoding the v1 branch. Every caller now passes its intended ref at the call site.

This is a pure refactor: all callers still pass the v1 branch ref. A subsequent commit switches the rewind picker to the topology read ref. The rename makes v1-pinning enforced by code at the resume.go fetch chain and explain.go blob prefetch sites, rather than relying on the implicit name.