Build for the merge from a copy its tests never touch, and keep or take only the same head's build

The gate and the repository's check mount the workspace read-write, so a
build from that tree could hold what the tests wrote under a key that
describes the merge. A check now clones the merge outside the workspace
before anything of the change runs, and builds from that. A check keeps,
and a walk takes, only a build an earlier check of the same pull request
at the same head made: another pull request's build, or an earlier
head's, is never registered. The walk reads the verdicts once
(novox/hq issue 703, review of #328).

cmd/mesh-controller/batches.go

@@ -688,7 +688,7 @@ func spelledOf(m inventory.BatchedMerge) string {
 func namedOf(m inventory.BatchedMerge, repository string) inventory.PlanMerge {
 	k := announcedOf(m)
 	return inventory.PlanMerge{Repository: repository, Commit: m.Commit, Number: k.Number, Title: k.Title, Moves: k.Moves,
-		Merged: m.Merged, Heard: m.Heard}
+		Head: k.HeadSHA, Merged: m.Merged, Heard: m.Heard}
 }
 
 // laterMerge says a was merged after b: by the forge's merge time, then by when each was heard.