R1:2bdcecc05daf

Help Mercurial build a parse trees with a few hundred items

Summary: Ref T5896. See that task and inline for a description.

Test Plan:
- Ran `bin/repository refs HGTESTX --trace`, saw sensible commands and a reasonable result.
- Faked the heads, set chunk size to 2, got this correct output from the algorithm:

> (((((((((1 or 2) or (3 or 4)) or ((5 or 6) or (7 or 8))) or (((9 or 10) or (11 or 12)) or ((13 or 14) or (15 or 16)))) or ((((17 or 18) or (19 or 20)) or ((21 or 22) or (23 or 24))) or (((25…