Keep a try off the forge and bound how many run (hq issue 686)

Review of #314: a try's start named its commit, which the forge's holder
matches to an open pull request's head and marks pending. Its start now
names no repository or commit. One try per commit and four in all, a try
never ages ahead of deliveries or pull request checks, and its id is
checked before it is a bus subject.

internal/link/builds.go

@@ -96,8 +96,27 @@ type BuildStart struct {
 	// an earlier taker left unfinished — most often a build agent replaced mid-build — and started
 	// again from the beginning (novox/hq issue 594). 0 from a machine that predates it.
 	Delivery int `json:"delivery,omitempty"`
+	// Try marks the start of a branch tried before its pull request (novox/hq issue 686), which names no
+	// repository and no ref (startOf).
+	Try bool `json:"try,omitempty"`
 }
 
+// startOf is what a build machine says of the ask it took. **A try's start names no repository and no commit**
+// (novox/hq issue 686): the forge's holder writes mesh/merge-gate pending on an open pull request whose head is
+// the commit a start names, and a try of that head is no check of the pull request — nothing on the forge moves
+// for a try. Its id, machine and time are all the queue reads of a start.
+func startOf(r BuildRequest, on string, delivery int, at time.Time) BuildStart {
+	s := BuildStart{ID: r.ID, Repository: r.Repository, Path: r.Path, Ref: r.Ref, On: on,
+		At: at.UTC().Format(time.RFC3339Nano), Delivery: delivery}
+	if IsTry(r) {
+		s.Repository, s.Path, s.Ref, s.Try = "", "", "", true
+	}
+	return s
+}
+
+// IsTry says an ask is a branch tried before its pull request (novox/hq issue 686).
+func IsTry(r BuildRequest) bool { return r.Check != nil && r.Check.Try }
+
 // Restarted is how a build handed out again says so (novox/hq issue 594), in its log and in its check's
 // report, so a second run never reads as the first. Neutral on why: a holder replaced mid-build, a paused
 // holder handing an ask back as it arrived, and a bus that held its acknowledgements past the wait all