bug#49731: 28.0.50; Filter xref results by filename

[Juri Linkov]

>>   / s   outline-show-by-heading-regexp
>>   / h   outline-hide-by-heading-regexp
>
> Thanks.  Does it make sense for these commands to follow similar
> semantics as Org Mode's sparse trees?  With C-c / / in an Org file, the
> entire buffer is folded as much as possible, but the matching items are
> made visible.

We don't have a goal to copy all Org Mode's features to Outline mode.
Only when someone really needs some Org Mode's feature, then patches
are welcome.

For example, very often I have the need to hide a large group of bodies
whose headings contain the same substring.  I guess you have the same need
since you created this feature request.  So this is implemented now.

> The main difference is that outline-show-by-heading-regexp would match
> only headings, but that is clear given the name of the command.

Indeed, the word 'heading' is added to the command names above
to distinguish them from possible future additions of more commands
that will hide by body regexps, or by both body and headings regexps.

Also the key prefix '/' will allow adding more related commands later
under the same key prefix like '/ /' for 'outline-occur', etc.