FR: revset function(s) to singularize revisions (maybe first(...) to start with)

[avamsi]

This could be useful with commands like rebase that only take a single revision -- for example,

$ jj rebase -b 'first(remote_branches("", origin).. ~ empty())' -d main

[yuja]

What do you expect to be the first with that revset?

I considered adding first() or limit() before, but rejected my idea because the order of revsets seemed to be implementation detail in jj, and the visible order could also be affected by #242.

It might be better to specify the sorting order explicitly (e.g. latest() to pick the first element in committer-timestamp order.)

[avamsi]

Makes sense, first was just the first singularizing function I could think of 😅 -- latest(...) might be even better for the rebase workflow I was thinking of (more often than not, I would probably be okay with just rebasing the latest commit I'm working on, leaving any older ones untouched).

Also, I just realized, the rebase example in my first comment should probably be more like the one below (exclude main: so that the command is repeatable) --

$ jj rebase -b 'latest(remote_branches("", origin).. ~ (main: | empty()))' -d main