pathutil: add doctests for canonpath()
This is a followup to
f445b10dc7fb. Since there's no way to ensure that more
drive letters than C: exist, this seems like the only way to test it. This is
enough to catch the
f445b10dc7fb scenario, as well as CWD outside of the repo
when the path isn't prefixed with path/to/repo.
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed