branchmap: stop looking for stripped branch
Since repoview in 2.5 we do not make special call to `branchmap` when stripping.
We just recompute the branchmap from a lower subset that still has valid
branchmap. So I'm dropping this dead code.
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program