bug#17559: 24.3.50; Partial completion does not complete underscore delimited input

[Stefan Monnier]

> Would something like the following be sufficient ?
[...]
> !           (let ((separator
> !                  (or (completion-metadata-get
> !                       (completion-metadata str table pred)
> !                       'completion-initials-separator)
> !                      (plist-get completion-extra-properties
> !                                 :completion-initials-separator)
> !                      "-")))

It could work, tho it requires changing all the corresponding completion
tables to provide the other "filling" when needed.
I think this should be fixable/fixed without needing to touch the
completion tables.

And if we add such `completion-initials-separator' properties, we'll be
stuck with them "for ever", so I'd rather not go there if there's
a chance we might not need them.


        Stefan