pathutil: add doctests for canonpath() stable

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.

--- a/mercurial/pathutil.py	Thu Nov 02 23:55:09 2017 -0400
+++ b/mercurial/pathutil.py	Fri Nov 03 22:22:50 2017 -0400
@@ -135,7 +135,47 @@
             return False
 
 def canonpath(root, cwd, myname, auditor=None):
-    '''return the canonical path of myname, given cwd and root'''
+    '''return the canonical path of myname, given cwd and root
+
+    >>> def check(root, cwd, myname):
+    ...     a = pathauditor(root, realfs=False)
+    ...     try:
+    ...         return canonpath(root, cwd, myname, a)
+    ...     except error.Abort:
+    ...         return 'aborted'
+    >>> def unixonly(root, cwd, myname, expected='aborted'):
+    ...     if pycompat.iswindows:
+    ...         return expected
+    ...     return check(root, cwd, myname)
+    >>> def winonly(root, cwd, myname, expected='aborted'):
+    ...     if not pycompat.iswindows:
+    ...         return expected
+    ...     return check(root, cwd, myname)
+    >>> winonly(b'd:\\\\repo', b'c:\\\\dir', b'filename')
+    'aborted'
+    >>> winonly(b'c:\\\\repo', b'c:\\\\dir', b'filename')
+    'aborted'
+    >>> winonly(b'c:\\\\repo', b'c:\\\\', b'filename')
+    'aborted'
+    >>> winonly(b'c:\\\\repo', b'c:\\\\', b'repo\\\\filename',
+    ...         b'filename')
+    'filename'
+    >>> winonly(b'c:\\\\repo', b'c:\\\\repo', b'filename', b'filename')
+    'filename'
+    >>> winonly(b'c:\\\\repo', b'c:\\\\repo\\\\subdir', b'filename',
+    ...         b'subdir/filename')
+    'subdir/filename'
+    >>> unixonly(b'/repo', b'/dir', b'filename')
+    'aborted'
+    >>> unixonly(b'/repo', b'/', b'filename')
+    'aborted'
+    >>> unixonly(b'/repo', b'/', b'repo/filename', b'filename')
+    'filename'
+    >>> unixonly(b'/repo', b'/repo', b'filename', b'filename')
+    'filename'
+    >>> unixonly(b'/repo', b'/repo/subdir', b'filename', b'subdir/filename')
+    'subdir/filename'
+    '''
     if util.endswithsep(root):
         rootsep = root
     else: