Remove most uses of detached instances in stocklines module

(The remaining use is in editing keyboard bindings; the linekeys module
also needs some work.)

Closes #294 on github.

quicktill/managetill.py

@@ -118,7 +118,6 @@ def keypress(self, k):
 
     def line_selected(self, kb):
         self.dismiss()
-        td.s.add(kb)
         if kb.stockline:
             stocklines.modify(kb.stockline)
         elif kb.plu:

quicktill/printer.py

@@ -298,24 +298,21 @@ def fits(s):
 
 
 def print_restock_list(printer, rl):
-    """
-    Print a list of (stockline,stockmovement) tuples.
+    """Print a list of (stockline,stockmovement) tuples
+
     A stockmovement tuple is (stockitem,fetchqty,newdisplayqty,qtyremain).
 
-    We can't assume that any of these objects are in the current
-    session.
+    All ORM objects must be attached to the current database session.
     """
     with printer as d:
         d.printline(f"\t{tillconfig.pubname}", emph=1)
         d.printline("\tRe-stock list")
         d.printline(f"\tPrinted {ui.formattime(now())}")
         d.printline()
         for sl, sm in rl:
-            td.s.add(sl)
             d.printline(f"{sl.name}:")
             d.printline(f"{sl.ondisplay}/{sl.capacity} displayed")
             for item, move, newdisplayqty, stockqty_after_move in sm:
-                td.s.add(item)
                 if move > 0:
                     d.printline(f" {move} from item {item.id} "
                                 f"leaving {stockqty_after_move}")