afaik, only merge commits have two parents. Then the source branch would still point to the head of that branch. Furthermore, without having checked, aren't merge commits deterministic in the order of references? I wouldn't be surprised if this information isn't trivially available in a porcelain command but more surprised if it's lost
If a branch is just a reference, then how that reference changes over time will be lost. As astrange says, that info will be in the reflog, but the reflog is subject to GC. Once a GC happens, that info is forgotten.
