Problem:
Re-editing a buffer (`:edit!`, re-reading a dir.lua buffer, etc.) drops
its `:bcd` directory, so the CWD falls back to the global one. Whereas
other buffer-local state (`b:` vars, local options) survives a reload.
Solution:
Don't clear buf dir in `buf_freeall()`; `do_ecmd()` calls that when
reloading/re-editing. `free_buffer_stuff()` still clears them when
a buffer is freed or reused for another file.
7e53d3ba4d