Fix RepoCommand to allow for relative URLs

Fix RepoCommand to allow for relative URLs

This is necessary for deploying submodules on android.googlesource.com.

* Allow an empty base URL. This is useful if the 'fetch' field is "."
  and all names are relative to some host root.

* The URLs in the resulting superproject are relative to the
  superproject's URL. Add RepoCommand#setDestinationURI to
  set this. If unset, the existing behavior is maintained.

* Add two tests for the Android and Gerrit case, checking the URL
  format in .gitmodules; the tests use a custom RemoteReader which is
  representative of the use of this class in Gerrit's Supermanifest
  plugin.

Change-Id: Ia75530226120d75aa0017c5410fd65d0563e91b
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>