fix(:bcd): do not "inherit" buffer-local dir

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`.

src/nvim/buffer.c

@@ -1778,7 +1778,7 @@ void set_curbuf(buf_T *buf, int action, bool update_jumplist)
   }
 
   // Maybe cd to buffer-local directory
-  fix_current_dir(false);
+  update_cwd(kCdCauseBuffer);
 }
 
 /// Enter a new current buffer.
@@ -2031,8 +2031,6 @@ buf_T *buflist_new(char *ffname_arg, char *sfname_arg, linenr_T lnum, int flags)
     trigger_undo_ftplugin(buf, curwin);
     // It's like this buffer is deleted.  Watch out for autocommands that
     // change curbuf!  If that happens, allocate a new buffer anyway.
-    // We also ask it to not free the buffer-local directory so we can reuse
-    // it.
     buf_freeall(buf, BFA_WIPE | BFA_DEL);
     if (aborting()) {           // autocmds may abort script processing
       xfree(ffname);
@@ -2162,8 +2160,6 @@ bool curbuf_reusable(void)
           && curbuf->b_ffname == NULL
           && curbuf->b_nwindows <= 1
           && !curbuf->terminal
-          && curbuf->b_localdir == NULL
-          && curbuf->b_prevdir == NULL
           && (curbuf->b_ml.ml_mfp == NULL || buf_is_empty(curbuf))
           && !bt_quickfix(curbuf)
           && !curbufIsChanged());