fix(cwd): nvim_win_set_buf of :bcd buf, changes caller CWD #41329

Problem:
Setting a :bcd buffer into another window, modifies the caller's CWD.

    local b = vim.api.nvim_create_buf(true, true)
    vim.api.nvim_buf_call(b, function() vim.cmd.bcd('..') end)
    vim.cmd('vsplit')
    vim.api.nvim_win_set_buf(vim.fn.win_getid(2), b)

    :echo haslocaldir(0) haslocaldir(-1,0) haslocaldir(-1,-1,0)
    0 0 0
    :echo getcwd() ==# getcwd(-1,-1)
    0

Analysis:
`ctx_dirs_save` only saves CWD if it predicts the switch can change it.
But win_set_buf() replaces the target window's buffer *after* the
switch, which cannot be "predicted" from `ctx_dirs_save`.

Solution:
Always snapshot whenever the switch enters another window.
Skipping `os_dirname` was a micro-optimization.

0 files changed, 0 insertions, 0 deletions