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.
#require unix-permissions no-root
$ hg init a
$ cd a
$ echo foo > b
$ hg add b
$ hg ci -m "b"
$ chmod -w .hg/store
$ cd ..
$ hg clone a b
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
new changesets 97310831fa1a
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ chmod +w a/.hg/store # let test clean up
$ cd b
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 1 changesets, 1 total revisions
$ cd ..