watchpr: arm the conflict rule on a new merge computation

A mergeable=false baseline meant a conflict could never be reported, since
Gitea sends false while it recomputes after a push. Arm on a head or base SHA
change as well as on a mergeable poll, break the conflict run on unknown and
failed polls, and print the baseline with the conditions it suppresses.

cmd/watchpr/main_test.go

@@ -182,3 +182,33 @@ func TestDescribeFailureNamesTheCause(t *testing.T) {
 		})
 	}
 }
+
+// A watch started against an already conflicted or CI-red PR sends no alert for
+// either, so the baseline line has to say that is what it is doing: before this,
+// such a run printed nothing in either direction.
+func TestBaselineLineNamesSuppressedConditions(t *testing.T) {
+	st := agent.PRState{
+		Ref:       agent.PRRef{Owner: "unkin", Repo: "repo", Number: 7},
+		State:     "open",
+		Mergeable: agent.MergeNo,
+		CIStatus:  "failure",
+		HeadSHA:   "cafebabecafebabe",
+		BaseSHA:   "deadbeefdeadbeef",
+	}
+	got := baselineLine(st)
+	for _, want := range []string{
+		"unkin/repo#7", "mergeable=false", "ci=failure", "head=cafebabe", "base=deadbeef",
+		"already non-mergeable", "CI already failure", "not alerting",
+	} {
+		if !strings.Contains(got, want) {
+			t.Errorf("baselineLine = %q, want it to mention %q", got, want)
+		}
+	}
+
+	clean := st
+	clean.Mergeable = agent.MergeYes
+	clean.CIStatus = "success"
+	if got := baselineLine(clean); strings.Contains(got, "not alerting") {
+		t.Errorf("baselineLine = %q, want no suppression note for a clean baseline", got)
+	}
+}