Preserve . semantics in Path.relative_to, closes #13310

Author: josevalim

lib/elixir/lib/path.ex

@@ -424,6 +424,8 @@ defmodule Path do
   defp relative_to_unforced(_, _, original), do: join(original)
 
   defp relative_to_forced(path, path, _original), do: "."
+  defp relative_to_forced(["."], _path, _original), do: "."
+  defp relative_to_forced(path, ["."], _original), do: join(path)
   defp relative_to_forced([h | t1], [h | t2], original), do: relative_to_forced(t1, t2, original)
 
   # this should only happen if we have two paths on different drives on windows

lib/elixir/test/elixir/path_test.exs

@@ -316,6 +316,8 @@ defmodule PathTest do
     assert Path.relative_to("./foo/../bar/..", File.cwd!()) == "."
 
     # both relative
+    assert Path.relative_to("usr/local/foo", ".") == "usr/local/foo"
+    assert Path.relative_to(".", "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(~c"usr/local/foo", "etc") == "../usr/local/foo"