bug#49731: 28.0.50; Filter xref results by filename

[Daniel Martín]

I plan to implement a new feature for xref, but I'd like to get some
opinions first:

Sometimes an xref backend returns a lot of results spread over several
files.  This usually happens in huge projects and for certain operations
like "search references".  To make them more manageable, I propose a new
command that can filter xref result groups (typically filenames) by a
regular expression.  A user could filter by "tests/", or something like
that, to only get results from unit tests.  If you want to see a similar
feature in action, go to
https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/background/background_contents.cc;bpv=1;bpt=1
and type on "Type to filter by file path", under the "References" tab.

Right now the only approach I know for this use case is to use Isearch,
but Isearch searches the entire xref buffer, including xref matches.

What do you think about this new feature? Do you have any suggestions
about how it should work?