Move get_build_desc_branch, improve stamp version

On further thought, it is obvious that get_build_desc_branch
belong on the Builder.

Similarly, on reflection it is obvious that the "stamp version"
stamp file for a "following" build tree should incorporate the
build description branch in its name
Towards making stamping consistent with "follow".
All tests passing again
Test unstamp and branches a little more
Improve the git.py docstring a bit