Before, every creation of `AnchorSeekTarget` called
`MultiBufferSnapshot::buffer_for_path`, which seeks the excerpts sum
tree. Now we avoid that by
- looking up the buffer snapshot for the anchor's buffer in the
`buffers` map (keyed by `BufferId`)
- handling the case where the anchor's buffer doesn't exist at the
original path key with a separate `AnchorSeekTarget::Missing` variant
We also added a separate optimization to `AnchorSeekTarget::cmp`, to
skip the full `PathKey` comparison when the cursor's `PathKeyIndex` is
the same as the anchor's. This should help with singleton multibuffers.
Self-Review Checklist:
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable
Release Notes:
- N/A
---------
Co-authored-by: Anthony Eid <hello@anthonyeid.me>
