Part of the `pending_change` closure in the `changetracking.prepare` was
a bit confusing because it has access to `bufnr` and `uri` but it could
actually contain pending changes batched for multiple buffers.
(We accounted for that by grouping `pending_changes` by a `uri`, but
it's not obvious what's going on)
This commit changes the approach to do everything per buffer to avoid
any ambiguity.
It also brings the debounce/no-debounce a bit closer together: The
only difference is now whether a timer is used or if it is triggered
immediately
074b033e7e
