revset: clarify heads() order doesn't matter while computing common ancestors Follows up 5460926352ee and 52f19a840543.

hghave: require clang-format >= 6 due to output change

cext: reformat with clang-format 6.0 It appears some changes in clang-format affect our code. I didn't dig into that deeper since the new output looks better.

py3: use bytes() to byte-stringify Abort message in handleremotechangegroup()

py3: don't str() to byte-stringify object in test-bundle2-remote-changegroup.t

py3: byte-stringify literals in extension in test-bundle2-remote-changegroup.t # skip-blame just some b''

py3: open file in binary mode in test-bundle2-remote-changegroup.t