I was wondering how they got along without generation numbers for so long. It was by comparing timestamps and those are unreliable because systems can be misconfigured. How they are going to handle legacy repos with that problem I still don't get. I am guessing that history is f'd.
New versions of git will actually go back and generate this information for old commits. This will lead to git being slightly slower when in old repositories until all the commits contain the generation information, but that should happen fairly quickly.
You just go the whole way up to the root node counting parents (taking max length when there are many routes), no problem with amiguity. The problem is - it's slow.
Not necessarily "all the commits", as I understand it (there's some debate in the thread, so I could be wrong) As long as at least one commit in each merged branch has a generation stored, it's simple to compute without going all the way back to all the roots.
