Problem:
Buffer-local CWD (:bcd) is "sticky", similar to window-local CWD (:lcd).
But this contradicts one of its main benefits: per-buffer "project root"
for LSP, OSC7.
Other problems:
- A buffer created with :edit/:enew/:new silently inherits b_localdir
(and b_prevdir) from the previous buffer.
- curbuf_reusable() refuses to recycle a scratch buffer that has
`b_localdir`.
- After :new/:vnew/:tabnew the CWD sticks to previous buffer's
`b_localdir` even though the new curbuf has none, so :new is not
equivalent to ":split | enew", and getcwd() disagrees with
haslocaldir().
- Requires "which buffer spawned this buffer" semantics that no other
buffer-local state has.
Solution:
Drop sticky/inherit behavior of buffer-local CWD (:bcd).
- do_ecmd: always apply the new curbuf's dir (`fix_current_dir`), like
`do_autochdir` already does. :tabnew from a :bcd buffer now reverts to
global CWD (and fires DirChanged), same as :tabnew from a :lcd window.
- curbuf_reusable(): recycling a scratch buffer frees its b_localdir.
To get sticky/inherit behavior of CWD, use `:lcd`.
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