refs/reftable: fix consistency checks with worktrees

refs/reftable: fix consistency checks with worktrees

The ref consistency checks are driven via `cmd_refs_verify()`. That
function loops through all worktrees (including the main worktree) and
then checks the ref store for each of them individually. It follows that
the backend is expected to only verify refs that belong to the specified
worktree.

While the "files" backend handles this correctly, the "reftable" backend
doesn't. In fact, it completely ignores the passed worktree and instead
verifies refs of _all_ worktrees. The consequence is that we'll end up
every ref store N times, where N is the number of worktrees.

Or rather, that would be the case if we actually iterated through the
worktree reftable stacks correctly. But we use `strmap_for_each_entry()`
to iterate through the stacks, but the map is in fact not even properly
populated. So instead of checking stacks N^2 times, we actually only end
up checking the reftable stack of the main worktree.

Fix this bug by only verifying the stack of the passed-in worktree and
constructing the backends via `backend_for_worktree()`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>