Fix "Argument list too long"

While using rbenv and commiting a submodule commit with a large number
of commits causes an "Argument list too long".

Instead of passing a large variable, pass in the ref and shell out to
git to append the submodule changes.

Change-Id: Iaa4981c3c1998174430e08a2c06fba9c4f70eee4
Reviewed-on: https://gerrit.causes.com/26338
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>
Reviewed-by: Lann Martin <lann@causes.com>
Reviewed-by: John Skopis <john.skopis@causes.com>
Tested-by: John Skopis <john.skopis@causes.com>