scmutil: change canonpath to use util.samefile (issue2167)
Fixes (on Windows in cmd.exe):
$ hg -R v:\x\a status V:\x\a\bar
abort: V:\x\a\bar not under root
where v:\x\a is a valid repository with a checked-out file "bar"
(Note the difference in casing: "v:\" versus "V:\")