util: use '\\' rather than using r'\'
We need bytes, and I find this just a little more immediately obvious
than doing rb'\'.
pycompat: introduce an alias for urllib.unquote
We have to use unquote_to_bytes on Python 3, so we need an abstraction
for this.
keyword: handle filectx _customcmp
Suggested by Yuya Nishihara:
https://www.mercurial-scm.org/pipermail/mercurial-devel/2016-October/089461.html
Related to
issue5364.