merge-slicing: introduce and use "inheritance point" for merge
The first part of the stable sorted list of revision of a merge will shared with
the one of others. This means we can reuse subranges computed from that point to
compute some of the subranges from the merge.
That point is latest point in the stable sorted list where the depth of the
revisions match its index (that means all revision earlier in the stable sorted
list are its ancestors, no dangling unrelated branches exists). This is a bit
expensive to find since we have to walk all the revision, but being able to
reuse subranges in all case (not just regular changesets) provide a massive
speedup so the cost is worth it.