submodule: document failure to handle relative superproject origin URLs

submodule: document failure to handle relative superproject origin URLs

This test case documents several cases where handling of relative
superproject origin URLs doesn't produce an expected result.

submodule.{sub}.url in the superproject is incorrect in these cases:
  foo
  ./foo
  ./foo/bar

The remote.origin.url of the submodule is incorrect in the above cases
and also when the superproject origin URL is like:
  foo/bar
  ../foo
  ../foo/bar

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>