bug#61340: 29.0.60; Extra space in xref buffer

[Juri Linkov <juri@linkov.net>]

>>> This was originally an effort to give the outputted text some "breathing
>>> room", and I think it looks a little better.
>> Indeed, it looks better but unfortunately at the cost of caused ambiguity.
>> I guess this is the reason why Grep and Occur don't add space.
>
> I suppose we could try to tone down the colon. Occur uses 'shadow' for both
> the number and the colon; Grep uses the color from 'default' (with an
> underline).
>
> If we fontify it with 'shadow' rather than 'xref-line-number', that might
> look a little better.

I already forgot that the default color is too glaring since I customized
it long ago to '(xref-line-number ((t (:inherit shadow)))).  But I also
customized other xref faces e.g. '(xref-file-header ((t (:extend t
:background "grey90")))), so these faces provide a better look.
OTOH, I'm not sure if the default of xref-line-number can be changed
because on the one hand the purple color from compilation-line-number
is needed in grep to make them more noticeable because the line numbers
are in the middle of the line unlike in occur.  But on the other hand
the current theme of xref buffers matches all colors of grep, so any
change will make their default themes different.