checkheads: add test where the rewrite of the other branch is not direct This will help testing that our logic is properly transitive.

check-heads: add tests about old heads indirectly pruned

checkheads: add more complexe case where a branch is split on multiple ones We extend case A-6 and A-7 with partial counterpart. These case are interesting because some of the partial pushing will (rightfully) works and some other won't.

checkheads: add a test of partially pushing a branch spread on multiple other If a branch is fully obsolete but is result are spread on multiple branch, pushing only one of them should detect we create new branches.