remotefilelog: check against bytes type instead of buffer and coerce to bytes
Fixes Python 3 compat here.
Differential Revision: https://phab.mercurial-scm.org/D5606
tests: add missing b prefixes in remotefilelog-getflogheads.py
# skip-blame just b prefixes
Differential Revision: https://phab.mercurial-scm.org/D5603