When committing, it was possible for the left-hand side multibuffer to
get the updated base text (via the `buffer_changed_since_sync`
mechanism) without updating its diff snapshot for that buffer (via the
diff subscription), causing a mismatch between that multibuffer's diff
state and its buffer state. The fix is to ensure for inverted diffs that
we always update pull an updated diff snapshot as part of
`sync_from_buffer_changes`.

This also removes some code that we added in #44838 to sync the
left-hand side multibuffer when edits on the right-hand side invalided
diff hunks. Instead, the left-hand side will just sync the next time the
diff recalculates when this happens, and will always consider hunks from
the last diff calculation as valid--so there will be a short window
where the diff transforms and `diff_hunks_in_range` don't match between
the two sides. That's okay because we don't rely on this in the display
map--the code that translates positions between the two sides accesses
the diff's `InternalDiffHunk`s directly rather than going through a
multibuffer API.

Release Notes:

- N/A