Directory name completion blocks when it shouldn't

[Tim Van Holder]

My original report received no replies and it's been over 3 months,
hence this ping.

Originally reported for GNU Emacs 22.0.50.1 of 2006-08-01.
Still present in GNU Emacs 22.0.91.1 of 2006-12-04.

Given a directory structure of the form:

/top-dir/sub-dir/a-file
/top-dir/some-file

With M-x cd, entering "/t" TAB completes to "/top-dir/" like it should,
but "/top-dir/s" TAB does NOT complete to "/top-dir/sub-dir/" even though
there is only 1 matching directory (TAB TAB shows only "sub-dir" as only
alternative); it's necessary to disambiguate the directory name from the
names of any and all files in the same location for tab completion to
proceed.  This seems like a bug.