py3: replace os.sep with pycompat.ossep (part 3 of 4)

py3: replace os.sep with pycompat.ossep (part 2 of 4) This part also replaces some chunks of os.sep with pycompat.ossep.

py3: replace os.sep with pycompat.ossep (part 1 of 4) os.sep returns unicodes on Python 3. We have pycompat.ossep which returns bytes. This patch is a part of 4 patch series which will replace all the occurrences of os.sep to pycompat.ossep

py3: replace os.pathsep with pycompat.ospathsep os.pathsep returns unicode on Python 3. We already have pycompat.ospathsep which return bytes on Python 3. This patch replaces all the occurrences of os.pathsep in the codebase (excluding tests) to pycompat.ospathsep.

py3: use %d instead of %s for integers Python 3 returns an error if we use %s as type specifiers for integers.

help: make multirevs just an alias for revsets The multirevs topis seems to be covered well by the revsets topic, so just make it an alias and remove multirevs.txt.

context: correct metadataonlyctx's parameter It's "originalctx", not "path" as Yuya pointed in [1]. [1]: www.mercurial-scm.org/pipermail/mercurial-devel/2016-December/091508.html

registrar: raise a programming error on duplicated registering Previous, registering different object with the same name would silently overwrite the first value with the second one. We now detect the situation and raise an error. No extension in test or core had the issues.