Comment by layer8

The issue is that if you consider a branch to be what is really the history of the branch tip, then a branch is not just the part starting from the last join with another branch. Instead it is some directed path through the commit DAG, a path that in general can’t be reconstructed from the information Git keeps.

If, for example, you have a structure like

        |
        o
       / \
      o   o
   A  |   |  B
      o   o
       \ /        
        o
       / \
      o   o
   C  |   |  D
      o   o
       \ /
        o
        |

then conceptually the path CA might be one branch and DB the other branch (or alternatively, CB and DA). But this is not something that is represented in Git’s model.