VCS: Do not try to add files that are already tracked

Task-number: QTCREATORBUG-2455
Change-Id: I4b1fa9676ad4a8d7293e035e4d6103251bef8aee
Reviewed-by: Nikita Baryshnikov <nib952051@gmail.com>
Reviewed-by: Cristian Tibirna <tibirna@kde.org>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Doc: reformat front page

Remove bullets from top-level items.

Change-Id: I1c8bbe55f2ce17163b6df6a75c50543e67b0d251
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>