In a similar spirit as the rework of retractboundary, the new algorithm is doing
an amount of work in the order of magnitude of the amount of changeset that
changes phases. (except to find new roots in impacted higher phases if any may
exists).

This result in a very significant speedup for repository with many old draft
like mozilla try.

runtime of perf:unbundle for a bundle constaining a single changeset (C code):

before 6.7 phase work: 14.497 seconds
before this change:     6.311 seconds (-55%)
with this change:       2.240 seconds (-85%)

Combined with the other patches that fixes the phases computation in the Rust
index, the rust code with a persistent nodemap get back to quite interresting
performances with 2.026 seconds for the same operation, about 10% faster than
the C code.