bug#63829: 29.0.90; project-find-file's future history breaks with common-parent-directory

[Juri Linkov]

> - The new option renamed to project-file-history-behavior with values t or
>   'relativize. I thought about removing it, but after all, the change is
>   a bit exotic, so there's bound to be people who would want to disable
>   it. And the new name is also more extensible (extra behaviors I could
>   think of by now: 'relativize-when-exists or 'separate -- the latter could
>   mean to use separate history var other than file-name-history). No hurry
>  to implement any of those, though.
> - project-or-external-find-file needs some special handling of the
>   relativization when external file names are chosen. Better solutions
>   welcome.
> - Announcement in NEWS. :-)

A typo in NEWS?  'relative' -> 'relativize'

Also to reduce confusion for everyone who will look at it,
better to rename the property to 'project-root' in:

    (propertize file 'project (project-root project))))

PS: The docstring mentions the limitation: "This only affects
history entries added by earlier calls to `project-find-file'".
There is no way to remove this limitation?  Maybe some clever way
to match every file name from the history against the list
of all known project roots to find the root on the file name
without property.  This will also work when the file history
is restored from the desktop file or by savehist.el.