Do not export TM_SCM_BRANCH

We originally exported this variable for the commit window (to show the branch name in the window title bar). However, the commit window no longer uses this, so stop exporting it.
Export TM_SCM_NAME based on attributes

We set the attribute scopes `attr.scm.*` in two different places in DocumentController (`updateExternalAttributes` and `scopeAttributes`); however, we only export TM_SCM_NAME if the scm variables are present. So if `scmStatus` is not enabled, TM_SCM_NAME is never set or exported, but `attr.scm.*` can still be present. Instead, use the externalScopeAttributes ivar since these should always be set.
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.