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

[Spencer Baugh <sbaugh@janestreet.com>]

BTW, one more feature in this vein (stealing this idea from Stefan)
would be if we automatically moved point to the same location in the
other file.  That might be a little too magical.  But it would be very
cool...

I guess the ideal thing we'd want is to move point to the same function,
which is a bit trickier...  could be done with imenu perhaps.  Not
sure...

Maybe the right call would be to have a keybinding in C-x p p like j or
something, which would just instantly jump you to the same file in the
other project.  So you'd just run C-x p p j and that would open the same
file in the other project, with point inside the same function (using
imenu), at the same offset in that function.

That could be helpful for other reasons too: I've often wanted "just put
me anywhere in this other project, I don't care where", and this could
be that command.  Although I suppose mostly I want that because C-x p p
isn't currently a generic prefix for any command, and if we convert it
to be that (with next-default-directory or something), I won't need
that.

Alternatively, maybe C-x p j could be an alternative to C-x p p, and
when it prompts for a project, it could prompt only for "sibling
projects" which have the same file structure.  And we could have a
built-in way to detect sibling projects: Any other worktree of the
current git repository is a sibling project.  (And we would make this
extensible too of course; maybe have both project-siblings and
vc-list-worktrees as extension points)