This PR makes the worktree's change events more useful in a few ways:

* The changes are now described by a cheaply clone-able collection, so
that they can be used in background tasks. Right now, I'm using a simple
Arc slice.
* The `UpdatedEntries` event now captures not only changes due to FS
changes, but also newly-loaded paths that are discovered during the
initial scan.
* The `UpdatedGitRepositories` event now includes repositories whose
work-dir changed but git dir didn't change. A boolean flag is included,
to indicate whether the git content changed.
* The `UpdatedEntries` and `UpdatedGitRepositories` events are now
*used* to compute the worktree's `UpdateWorktree` messages, used to sync
changes to guests. This unifies two closely-related code paths, and
makes the host more efficient when collaborating, because the
`UpdateWorktree` message computation used to require walking the entire
`entries` tree on every FS change.