height N, we default to creating a new file at level N + 1. We then decide to
merge with the Nth level if one of two conditions hold:
- 1. The expected file size for level N + 1 is at least half the file size for
- level N.
+ 1. `--size-multiple=<X>` is specified or X = 2, and the number of commits in
+ level N is less than X times the number of commits in level N + 1.
- 2. Level N + 1 contains more than 64,0000 commits.
+ 2. `--max-commits=<C>` is specified with non-zero C and the number of commits
+ in level N + 1 is more than C commits.
This decision cascades down the levels: when we merge a level we create a new
set of commits that then compares to the next level.