In SubmitFileModel, filetype icons are missing for not "overlaid" icons in
Core::FileIconProvider. This is fixed by ensuring valid file paths are passed
to QFileInfo. VcsBase::SubmitFileModel has now a "repositoryRoot" property
so it can constructs absolute file paths.

VcsBaseSubmitEditor::setFileModel() now relies on SubmitFileModel::repositoryRoot()
instead of passing this info as annex parameter.

Change-Id: Ie045c3a7b247a5b5d48d10aaaf67cb3fdcf9fa7e
Reviewed-by: Orgad Shaneh <orgads@gmail.com>