feat(editor): :bcd changes buffer-local directory

Problem:
No way to set a buffer-local directory.
Use-cases:
- "Root dir" for LSP (and the "project" concept).
- `:terminal` OSC 7

Solution:
Add `:bcd` command.

- Extend `getcwd()` to take a third arg; `getcwd(-1, -1, bunfr)` returns
  the buffer-local working directory.
- Buffer-local directories have less priority than window-local
  ones, and more priority than tab-local ones.

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

src/nvim/buffer.c

@@ -938,6 +938,9 @@ bool buf_freeall(buf_T *buf, int flags)
   }
   syntax_clear(&buf->b_s);          // reset syntax info
   buf->b_flags &= ~BF_READERR;      // a read error is no longer relevant
+
+  XFREE_CLEAR(buf->b_localdir);
+  XFREE_CLEAR(buf->b_prevdir);
   return true;
 }
 
@@ -1023,6 +1026,8 @@ static void free_buffer_stuff(buf_T *buf, int free_flags)
   map_clear_mode(buf, MAP_ALL_MODES, true, false);  // clear local mappings
   map_clear_mode(buf, MAP_ALL_MODES, true, true);   // clear local abbrevs
   XFREE_CLEAR(buf->b_start_fenc);
+  XFREE_CLEAR(buf->b_localdir);
+  XFREE_CLEAR(buf->b_prevdir);
 
   buf_free_callbacks(buf);
 }
@@ -1771,6 +1776,9 @@ void set_curbuf(buf_T *buf, int action, bool update_jumplist)
   if (bufref_valid(&prevbufref) && prevbuf->terminal != NULL) {
     terminal_check_size(prevbuf->terminal);
   }
+
+  // Maybe cd to buffer-local directory
+  fix_current_dir(false);
 }
 
 /// Enter a new current buffer.
@@ -2023,6 +2031,8 @@ 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);
@@ -2152,6 +2162,8 @@ 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());