In that diagram, I see 2 sequences of commits (two branches).
The 'original branch' (e.g. "unstable") goes: 0,1,2,5,6
Then there is a topic branch (e.g. "add-frob"), which goes: (0,1),2,3,4,(5). Note that I consider the the 'add-frob' branch ended at the merge commit, so there is no "revno 6 of 'add-frob').
I don't consider that merging 'add-frob' back into unstable means that "revno 2 in unstable" could mean commit D - I would call commit D "revno 2 in add-frob".
The 'original branch' (e.g. "unstable") goes: 0,1,2,5,6
Then there is a topic branch (e.g. "add-frob"), which goes: (0,1),2,3,4,(5). Note that I consider the the 'add-frob' branch ended at the merge commit, so there is no "revno 6 of 'add-frob').
I don't consider that merging 'add-frob' back into unstable means that "revno 2 in unstable" could mean commit D - I would call commit D "revno 2 in add-frob".
Does that system work?