util.pathto: return '.' instead of an empty string

This could happen with something like
pathto(repo.root, 'foo/bar', 'foo/bar')

mercurial/util.py

@@ -335,7 +335,7 b' def pathto(root, n1, n2):'
         a.pop()
         b.pop()
     b.reverse()
-    return os.sep.join((['..'] * len(a)) + b)
+    return os.sep.join((['..'] * len(a)) + b) or '.'
 
 def canonpath(root, cwd, myname):
     """return the canonical path of myname, given cwd and root"""