All the callsites of this method have access to the repo, and I'd
rather not have to duplicate this across alternative bmstore
implementations. Besides, it feels like a bit of a layering violation.
`mercurial.bookmarks.bmstore` no longer has a convenience method for looking up changectx instances from a bookmark name. Use `repo[repo.bookmarks[name]]` intead of `repo.bookmarks.changectx(name)`.