Play nice with other source control providers

Hide the GitDiffMargin UI for files which are not obviously within a Git repository. This allows other SCC implementations to provide a similar feature without continually increasing the width of the margins.

The current implementation of this test means the margin is disabled for newly created files in the editor, but this is OK at least for now since `GitCommands.GetGitDiffFor` returns an empty collection for files which have not been committed at least once.