refactor(undo): redundant "forget" logic #41980

Problem:
`u_undo_and_forget()` and `u_forget_unchanged()` do similar undo-tree
surgery: unlink a header, promote the alt branch it displaced.

Solution:
Extract common logic to `u_forget_header()`.

Bonus:
- The `:undo!` copy now fixes `b_u_oldhead`, so after forgetting a root
  header, its alt branch is still listed by `undotree()`.
- Freeing can no longer unlink the promoted branch: `uh_alt_prev` is
  cleared first. The old code relied on the forgotten header being the
  first sibling.

0 files changed, 0 insertions, 0 deletions