Deeply nested revset crashes jj with a stack overflow

In any repo:

$ jj log -r "root()$(python3 -c "print('-'*1000)")"
thread 'main' (4221516) has overflowed its stack
fatal runtime error: stack overflow, aborting

(Debug build of main @ f2bdea4b1; a release build will need a deeper expression, but the recursion is unbounded so some depth always crashes. I ran this on MacOS but it shouldn't be platform specific)

This is the same crash class as #4031, but the fix there (415c831e3) flattens union nodes specifically; every other nestable operator still constructs an unboundedly deep tree.

I found this while porting over the proptest tests to Hegel (sorry I'm so delayed on that. Ran into some issues doing it and then other stuff got on top of me).