When renaming a reference, we need to iterate over every HEAD and
potentially update it in case it is a symbolic reference pointing to the
previous name of the renamed reference. Most importantly, this doesn't
only include HEADs from the repo we're renaming the reference in, but we
also need to iterate over HEADs from linked worktrees.

In order to update the HEADs, we directly read them from the worktree's
gitdir and thus assume that both repository and worktrees use the
filesystem-based reference backend. But this breaks as soon as one got a
repository with a different refdb and breaks our own abstractions. So
let's instead update HEAD references via the refdb by first opening each
worktree as a repository and then using the usual functions to read and
update HEADs. This is a lot less efficient than the current code, but
it's not like we can really help this: going via the refdb is mandatory.