bug#68958: [PATCH] Support bookmarking Xref results buffers

[Juri Linkov]

> Also, we'd probably want to limit the size of the printed representation
> (FETCHER created by project-find-regexp currently closes over the full list
> of files, that's too much to save; it will probably be a good idea to
> rewrite it to fetch the list of files anew).

Indeed, I confirm the problem: reverting an xref buffer with 'g'
can't find matches in a new file added after the first run.

Doesn't this hint that both a bookmark and a revert function
should store more high-level arguments?  For example, for
'project-find-regexp' it should be sufficient to store
its argument 'regexp' together with 'default-directory'
to reconstruct the previous xref buffer contents.
Then it could store these arguments in a buffer-local variable
that could be used to create a bookmark.

PS: With C-u 'project-find-regexp' reads more arguments
in the body, but such reading should be moved to the
interactive form anyway like 'rgrep' does.