bug#74616: 30.0.92; tmm always displays the *Completions* buffer

[Daniel Mendler]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> Okay. Can you please clarify why you assume that overriding isn't
>> reliably possible if the metadata is specified directly?
>
> Because `completion-category-overrides` works only if there's an
> appropriate `category` to use.  If the `eager-display` info comes
> straight from the metadata, there's no guarantee that there's
> an associated `category` info that can be used.

I've added both completion categories and eager-display. Please see the
attached patch. As soon as there is a completion category, overriding
will work. While the completion category is a necessary prerequisite to
support overriding, adding eager-display at either places should do.
Also note the addition of the completion-eager-display defcustom (with
values nil, t, auto), which will make it much easier to turn off eager
display globally.

Daniel