Make `scm_enabled_for_path` API public
Let user know if scm status is disabled

Currently, the only way a user would now if the scm status was disabled is the lack of the status icons in the file browser. Even worse, when using ⇧⌘Y we would show the status as "Unversioned". This is confusing. So now we explicitly check if the scm status is disabled and inform the user.
Move `scm_enabled_for_path` function below `root_for_path`

This is to keep with the original organization of the code where the public APIs are defined after the private functions and in the order declared in the header.