Leave a module out of a walk when its check built it the same as what runs (hq issue 721)

A one-sentence change in a package mesh-builder imports but never reaches moved
build-agent. A merge check now compares each module it builds with the build the
mesh holds, the whole image without file times, labels or creation time, and a
walk is cut without a module every check of its pull requests found the same,
while the mesh still holds that build. A comparison that fails moves the module
as before.

cmd/mesh-builder/main.go

@@ -525,7 +525,7 @@ func checkBuiltOf(built []builder.CheckBuilt) []link.CheckBuilt {
 	var out []link.CheckBuilt
 	for _, b := range built {
 		c := link.CheckBuilt{Module: b.Module, Key: b.Key, NotBuilt: b.NotBuilt, Took: b.Took.Round(time.Second).String(),
-			Kept: b.Kept}
+			Kept: b.Kept, Same: b.Same, SameAs: b.SameAs}
 		for _, m := range b.Made {
 			c.Made = append(c.Made, link.MadeArtifact{Name: m.Name, Kind: m.Kind, Reference: m.Reference})
 		}