fix(dir): let 3P dir-browser plugin rename the dir buffer #40529

Problem:
`FileType directory` fires *before* the `BufEnter` path where `dir.lua`
populates the buffer, in order to allow a 3P dir-browser plugin to handle the
event and (optionally) rename the buffer to e.g. `example:///tmp/foo/`.

But currently, `open_buffer()` continues after `filetypedetect BufRead` as if
the original directory buffer is *still* current and valid, which may fall
through to the built-in `nvim.dir` path after a 3P plugin ALREADY handled the
directory.

Solution:
Keep a buf ref around the early `filetypedetect BufRead` call for directory
buffers.

If the `FileType directory` handler switches away from/deletes the original
directory buffer, stop the open path instead of continuing into the built-in
`nvim.dir` flow.

src/nvim/buffer.c

@@ -413,7 +413,12 @@ int open_buffer(bool read_stdin, exarg_T *eap, int flags_arg)
       && after_pathsep(curbuf->b_ffname,
                        curbuf->b_ffname + strlen(curbuf->b_ffname))) {
     if (augroup_exists("filetypedetect")) {
+      bufref_T bufref;
+      set_bufref(&bufref, curbuf);
       do_doautocmd("filetypedetect BufRead", false, NULL);
+      if (!bufref_valid(&bufref) || curbuf != old_curbuf.br_buf || aborting()) {
+        return FAIL;
+      }
     }
   }