N-bisect: parallel bisection

Instead of testing one commit per round, pick N evenly-spaced commits
and test them all at once.

With n=7 and 1000 commits you're looking at ~4 rounds instead of 10.
Git has the primitives for this (git rev-list --bisect-all, etc.).

Depends on having decent build caching in place, otherwise you're
just burning resources. Guillaume noted this was considered before
for KernelCI and wasn't worth it without caching.

Existing tools: git-pisect (Perl), risect (Rust).

The issue names no files or tests. Begin by evaluating git rev-list --bisect-all and the build-caching dependency, then compare the approach with git-pisect and risect; done means supporting cached, parallel N-way commit testing without wasting resources.

Written by the indexing model from the issue text.