sync: Warn when a remote's export is not updated to the current tree because export tracking is not configured.

Only display the warning when the current branch has a tree that is not
the same as the tree in the export.

Note that it doesn't check to see if the current tree is
in incompleteExportedTreeish; it might be worth checking that and reminding
the user about an incomplete export, but when export tracking is not
configured, they are probably not in the right clone of the repository to
resolve the incomplete export.

This commit was sponsored by Ethan Aubin.

Assistant/Threads/Exporter.hs

@@ -64,6 +64,7 @@ exportToRemotes rs = do
 			Annex.changeState $ \st -> st { Annex.errcounter = 0 }
 			start <- liftIO getCurrentTime
 			void $ Command.Sync.seekExportContent rs
+				=<< join Command.Sync.getCurrBranch
 			-- Look at command error counter to see if the export
 			-- didn't work.
 			failed <- (> 0) <$> Annex.getState Annex.errcounter