debugsparse: abort if the repository is not sparse instead of ui.status()

This is similar to what narrow extension does.

Differential Revision: https://phab.mercurial-scm.org/D6149

hgext/sparse.py

@@ -318,9 +318,10 b' def debugsparse(ui, repo, *pats, **opts)'
             if temporaryincludes:
                 ui.status(_("Temporarily Included Files (for merge/rebase):\n"))
                 ui.status(("\n".join(temporaryincludes) + "\n"))
+            return
         else:
-            ui.status(_('repo is not sparse\n'))
-        return
+            raise error.Abort(_('the debugsparse command is only supported on'
+                                ' sparse repositories'))
 
     if include or exclude or delete or reset or enableprofile or disableprofile:
         sparse.updateconfig(repo, pats, opts, include=include, exclude=exclude,