#63145 exposed a bug in the the `snapshot_with_edits` and `fast_forward`
methods that we use to evolve the base text buffer for diffs: we
preserved the value of the `tree_sitter_data` field from the old
snapshot. That causes any diff whose base text has changed to have stale
bracket ranges and now stale syntax highlighting as well, which can
manifest as unhighlighted text in the deleted portion of an expanded
diff hunk.

This PR fixes the issue by clearing the `tree_sitter_data` field in
`snapshot_with_edits`.

Release Notes:

- N/A (problem exists in nightly only)