branchmap: revert
c34532365b38 for Python 2.7 compatibility
Old versions of python 2.7 don't like that the second argument to
struct.unpack_from is a bytearray, so the change removing the util.buffer
around that argument in branchmap broke running on older versions of python
2.7.
Differential Revision: https://phab.mercurial-scm.org/D330
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