Fix CI failure on latest stable Neovim: jump list not updated from nofile buffer

[Copilot]

In newer Neovim stable, :edit executed from a nofile buffer no longer adds the current position to the jump list automatically, breaking g:fern#keepjumps_on_edit = 0 behavior.

Changes

• autoload/fern/internal/buffer.vim: When opener='edit' and keepjumps=0, explicitly record the current position via normal! m' (sets previous context mark → adds to jump list) before executing the edit command. Always apply the keepjumps modifier to prevent double-add:
  • keepjumps=0: position recorded by m', keepjumps edit prevents duplicate
  • keepjumps=1: no m', keepjumps edit intentionally skips recording

" Before
if options.keepjumps && options.opener ==# 'edit'
  let options.mods .= ' keepjumps'
endif

" After
if options.opener ==# 'edit'
  if !options.keepjumps
    execute "normal! m'"  " explicitly record position; :edit no longer does this in newer Neovim
  endif
  let options.mods .= ' keepjumps'  " always prevent :edit from double-adding
endif

• test/behavior/jumplist.vimspec: Removed comments misattributing the jump count difference to -wait side effects (the actual cause was G on nofile adding one entry, and edit adding another — the latter now handled explicitly via m').