Always return a "vaild" scm URL when requested

Previously when constructing a scm URL from a path, if scm was not enabled
we returned a file URL. This could result in some unexpected behavior.
For instance, when a directory was selected in the file browser,
activating "SCM Status" would just open the untracked directory.

Instead, we will move the check to see if the path is being tracked when
creating the scm datasource. If it is not being tracked, we give some additional
feedback in the file browser.