vcsbase: throw away VCSJobRunner, replaced by GitCommand

VCSBase::Command was created from GitCommand, it replaces VCSJobRunner.
VCSBase::Command is simpler to use (in VCSBaseClient) and commands
can show up in the progress manager.
From now on, the git plugin can easily rely on vcsbase.

Change-Id: I40593f354249cd94a4c79e90f9bf1362ff3bf5ec
Merge-request: 394
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>
Reviewed-on: http://codereview.qt-project.org/6078