Share one prefix between the no-move Why's writer and its reader (review of #309, issue 648)

cmd/mesh-controller/release_plan.go

@@ -253,12 +253,16 @@ func gates(p inventory.Plan, edges []inventory.Edge, rollsOut func(string) bool)
 
 // noMoveWhy is what a plan says of a module of its tier whose build was made from the source every machine
 // running it runs (novox/hq issue 280): nothing is sent for it.
-const noMoveWhy = "no move: made from the source every machine running it runs"
+// noMovePrefix opens it, and is what noMove reads: one prefix for the writer and the reader.
+const (
+	noMovePrefix = "no move"
+	noMoveWhy    = noMovePrefix + ": made from the source every machine running it runs"
+)
 
 // noMove is whether the plan marked the module no move and sent it nothing. Read from what the plan says,
 // so a plan kept before the mark had a name of its own is read the same (novox/hq issue 648).
 func noMove(s *inventory.PlanModule) bool {
-	return s != nil && s.FirstAt == nil && s.Gate == nil && strings.HasPrefix(s.Why, "no move")
+	return s != nil && s.FirstAt == nil && s.Gate == nil && strings.HasPrefix(s.Why, noMovePrefix)
 }
 
 // applied says whether every machine running the module has reported since the module was built.