bug#34908: Push mark in xref-push-marker-stack

[Juri Linkov]

>> I know there is ‘M-,’ but why not allow using the standard keys
>> ‘C-x C-SPC’ (pop-global-mark) and ‘C-u C-SPC’ (in the same file)
>> as well?
>
> IMO that separation of marks between local and global ones, and navigation
> between them (where you have to remember whether your previous navigation
> was between files or inside one file) is extremely counter-intuitive, so
> I don't have a lot of experience with that facility.

Yes, its inconvenience is that you have to remember whether a previous
position was in the same file or not, and depending on this decide
what command to use: global or local pop.

> Even so, I think it's been nice enough that every command can choose
> whether it pushes the mark to the local/global buffer rights, and/or it
> adds it to the xref marker stack. Do we have any particular guidelines in
> the manual for when either should happen?

I think it should push to both.

> xref-push-marker-stack is used externally as well as a replacement for
> find-tag-marker-ring (which is now marked obsolete). And any command that
> replaced the usage of the latter with the former, and also intended to push
> mark, probably does as a separate action.

> The closer counterpart to find-tag-in-order calling push-mark would be to
> add that call at the beginning of xref--show-xrefs.

I'm not sure where to call push-mark: closer to the command,
or closer to ring-insert.  It seems a suitable place for
push-mark is in xref-push-marker-stack as its name suggests.