checkheads: add some extra tests about "partial push" This adds a couple of test that checks that the head replacement code is properly ignored replacement not relevant to the push.

checkheads: handle partial obsolescence We now properly detects situations were only parts of the remote branch is obsoleted. To do so, we process children in the branch recursively to see if they will be obsolete. The current code has some trouble when the remote branch in unknown locally, or when the prune happened on a successors that is not relevant to the push. These case will be handled later. The processing code is becoming more and more complex, a lighter approach would be to check for the obsolescence markers that are relevant to the pushed set, but I prefer to stick with the current approach until more test cases are written.