- If the submodule doesn't eixst yet, then submodule update is called
- If the submodule points at the right commit then nothing happens.
- If the submodule is pointed at any other commit, this script will attempt to rebase.
  - If the rebase fails (for any reason!) then the local HEAD is used rather than just switching to whatever upstream uses.

All automatic updating is disabled by default.
Set submodule.update=true in private.properties to enable.

Change-Id: I9f370a33001bbfea5079687a41cc9cbfa07eea33
Reviewed-on: http://gerrit.dmdirc.com/326
Reviewed-by: Gregory Holmes <greboid@dmdirc.com>
Tested-by: Gregory Holmes <greboid@dmdirc.com>