bug#6830: widget-complete bad completions in :type 'file

[Drew Adams]

> If I type a file name and then do `M-x widget-complete', 
> exactly as the bug recipe says, then, just as the bug report
> says, I get all of the files in the current directory as
> completion candidates.
> 
> E.g. with `foo' as the text in the field, left justified, no spaces...

Similarly, for an absolute file name, such as `c:/foo'.  With point at the end
(regardless of the absolute file-name text preceding point), the completion
candidates are all of the files in the current directory, not files c:/.

If I had to guess (and guessing is all that's possible - no help available,
AFAICT), I'd guess that the text before point is ignored, so the empty input ""
is completed in `default-directory', since no absolute file name is found
_after_ point.  Most unusual behavior, in any case.

Wrt there being no help: There is a tooltip, but it says only this:
"M-TAB: complete field; RET: enter value"

Hardly helpful in trying to understand the complex completion behavior.