--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -314,11 +314,16 @@ def globre(pat, head='^', tail='$'):
 
 _globchars = {'[': 1, '{': 1, '*': 1, '?': 1}
 
-def pathto(n1, n2):
+def pathto(root, n1, n2):
     '''return the relative path from one place to another.
+    root should use os.sep to separate directories
     n1 should use os.sep to separate directories
     n2 should use "/" to separate directories
     returns an os.sep-separated path.
+
+    If n1 is a relative path, it's assumed it's
+    relative to root.
+    n2 should always be relative to root.
     '''
     if not n1: return localpath(n2)
     a, b = n1.split(os.sep), n2.split('/')