Use the new Path.relative_to and ensure it won't fail for equals paths

Author: José Valim

lib/elixir/lib/exception.ex

@@ -176,7 +176,7 @@ defmodule Exception do
       file = to_binary(file)
 
       if cwd do
-        file = :binary.replace(file, cwd <> "/", "")
+        file = Path.relative_to(file, cwd)
       end
 
       if line && line != 0 do

lib/elixir/lib/path.ex

@@ -120,8 +120,8 @@ defmodule Path do
     relative_to(t1, t2, original)
   end
 
-  defp relative_to(t1, [], _original) do
-    FN.join(t1)
+  defp relative_to([_|_] = l1, [], _original) do
+    FN.join(l1)
   end
 
   defp relative_to(_, _, original) do

lib/elixir/test/elixir/path_test.exs

@@ -57,6 +57,7 @@ defmodule PathTest do
     assert Path.relative_to("/usr/local/foo", "/usr/local") == "foo"
     assert Path.relative_to("/usr/local/foo", "/") == "usr/local/foo"
     assert Path.relative_to("/usr/local/foo", "/etc") == "/usr/local/foo"
+    assert Path.relative_to("/usr/local/foo", "/usr/local/foo") == "/usr/local/foo"
 
     assert Path.relative_to("usr/local/foo", "usr/local") == "foo"
     assert Path.relative_to("usr/local/foo", "etc") == "usr/local/foo"