relink: abort earlier when on different devices (issue3916)

Add a first check of the devices before collecting candidate files. This is
much quicker when big repos are on different devices.
Keep the existing check in prune. It checks for same device of the files. This
could probably be different in a special repo store (with symlinks).

hgext/relink.py

@@ -48,6 +48,10 b' def relink(ui, repo, origin=None, **opts'
         ui.status(_('there is nothing to relink\n'))
         return
 
+    if not util.samedevice(src.store.path, repo.store.path):
+        # No point in continuing
+        raise util.Abort(_('source and destination are on different devices'))
+
     locallock = repo.lock()
     try:
         remotelock = src.lock()