Say an unanswered merge check in its own words, with no action it cannot take (issue 438)

mesh-delivery now says a head whose merge check started and never answered as a
stalled line in state unanswered. Through the generic delivery words it would read
as a delivery and offer a Stop that mesh-delivery refuses, since no delivery of
the head exists; it now names the checks waited on and what the operator can do.

cmd/mesh-controller/delivery_conditions.go

@@ -37,10 +37,13 @@ type stalledLine struct {
 	// Number is the pull request's, for a line of a head the forge never announced (novox/hq issue 347).
 	Number int    `json:"number,omitempty"`
 	State  string `json:"state"`
-	For    string `json:"for"`
-	Bound  string `json:"bound"`
-	H2     string `json:"h2"`
-	Says   string `json:"says"`
+	// Waiting are the merge checks a line of the state `unanswered` waits on, as mesh-delivery says each:
+	// "mesh/merge-gate pending since <UTC time>", "mesh/repo-check never set" (novox/hq issue 438).
+	Waiting []string `json:"waiting,omitempty"`
+	For     string   `json:"for"`
+	Bound   string   `json:"bound"`
+	H2      string   `json:"h2"`
+	Says    string   `json:"says"`
 }
 
 // operatorsOnly is whether the table leaves H2 nothing to do for the line: the state is the operator's.