FR: Revset selecting multiple fork points

[DavidDTA]

Is your feature request related to a problem? Please describe.
In my jj log, I would like to show the set of commits I'm interested in x, along with the fork point between each commit in x and trunk().

Describe the solution you'd like
One option would be a fork_points(revset, reference) function, which would select exactly the set of commits I've described.

Describe alternatives you've considered
There could be a function like for_each(revset, lambda) which would create a revset containing the union of the revsets obtained by running the lambda on each revision in revset. This would allow the above function to be defined as fork_points(revset, reference) = for_each(revset, |revision| fork_point(revision, reference)). This would add quite a bit of complexity to the language, and would also invite another variant that takes the intersection instead of the union of the resulting revsets. Alternatively, an even more generic solution would be to introduce fold.

Additional context
I don't believe this is currently possible, but please let me know if it is! I am currently using ancestors(trunk()..x, 2) & ::trunk(), but this selects some undesirable revisions in some cases where trunk()..x contains a merge commit.

[martinvonz]

but this selects some undesirable revisions in some cases where trunk()..x contains a merge commit.

Can you give an example?

[DavidDTA]

Simplest example is:

$ jj config set --repo 'templates.log' 'change_id.shortest(0)' 
$ jj log -r 'all()'                                           
@  wo
│ ○  r
╭─┤
○ │  t
├─╯
○  w
◆  z
$ jj log -r 'fork_point(r|wo)'                           
○  t
│
~
$ jj log -r 'ancestors(wo..r, 2) & ::wo'                             
○  t
○  w
│
~