tools/bump: bisect go.mod one module at a time

golang.org/x/net deprecated http2.Server, staticcheck failed on
hscontrol/noise.go, and the whole batch of 50 direct requirements was
dropped for it. The bisect tries every atom together first, so one atom
per module costs nothing until something actually breaks.

tools/bump/bisect.go

@@ -76,7 +76,11 @@ func applySet(ctx context.Context, r *repo, atoms []atom) ([]string, error) {
 			return nil, fmt.Errorf("%s: %w", a.Name, err)
 		}
 
-		summaries = append(summaries, summary)
+		// An atom that found nothing to do reports no summary; listing it
+		// would fill the report with modules that did not move.
+		if summary != "" {
+			summaries = append(summaries, summary)
+		}
 	}
 
 	err := settle(ctx, r)