case-insensitive filename completion bug

[Miles Bader <miles.bader@necel.com>]

If I have filename called "Foo", and I try to visit it with find-file,
and have `read-file-name-completion-ignore-case' set to t, then I can
type:

   C-x C-f f o TAB

hitting TAB will (1) change the lowercase "f" I typed to be "F", and (2)
insert the missing "o".

However, if I type:

   C-x C-f f o o TAB

it will say " [sole completion]" -- but will _not_ change change the
lowercase "f" to uppercase.  Because I'm using a case-sensitive
filesystem, the final filename doesn't exist, even though emacs said it
did.

One of the main reasons I like case-insensitive completion is that Emacs
will usually take care of capitalizating of studly-caps filenames; as a
result, I instinctively hit TAB to "correct" what I just typed and then
hit RET.  So this corner case where TAB _doesn't_ do it is pretty
annoying (and occurs surprisingly often).

So I think Emacs should replace the entered name with the "correct"
completion even if they are the same length.

[This case seems to work correctly for other types of case-insensitive
completion, e.g., buffer-names.]

Thanks,

-Miles

-- 
If you can't beat them, arrange to have them beaten.  [George Carlin]