dashboard/app: dedup changes in UI

Currently for every benchmark/metric we show all changes for all builders x procs.
With 4 builders and 5 procs, that's 20 changes (20 red/green boxes in a single cell).
Instead show only maximum change for every benchmark/metric.
This significantly reduces clutter in UI.
When you click on the red/green box, you can see the rest of the changes.

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/126430043

dashboard/app/build/perf_changes.go

@@ -132,6 +132,7 @@ func handleOneCommit(pc *PerfConfig, com *Commit, rc *PerfResultCache, baseRes *
 			}
 		}
 		changes := significantPerfChanges(pc, builder, res0, res1)
+		changes = dedupPerfChanges(changes)
 		for _, ch := range changes {
 			v := new(perfChangesChange)
 			v.Builder = builder
@@ -165,6 +166,34 @@ func handleOneCommit(pc *PerfConfig, com *Commit, rc *PerfResultCache, baseRes *
 	return uiCom, nil
 }
 
+// Find builder-procs with the maximum absolute diff for every benchmark-metric, drop the rest.
+func dedupPerfChanges(changes []*PerfChange) (deduped []*PerfChange) {
+	maxDiff := make(map[string]float64)
+	maxBench := make(map[string]string)
+	// First, find the maximum.
+	for _, ch := range changes {
+		bench, _ := splitBench(ch.Bench)
+		k := bench + "|" + ch.Metric
+		v := ch.Diff
+		if v < 0 {
+			v = -v
+		}
+		if maxDiff[k] < v {
+			maxDiff[k] = v
+			maxBench[k] = ch.Builder + "|" + ch.Bench
+		}
+	}
+	// Then, remove the rest.
+	for _, ch := range changes {
+		bench, _ := splitBench(ch.Bench)
+		k := bench + "|" + ch.Metric
+		if maxBench[k] == ch.Builder+"|"+ch.Bench {
+			deduped = append(deduped, ch)
+		}
+	}
+	return
+}
+
 func findMetric(c *perfChangesCommit, metric string) *perfChangesMetric {
 	for _, m := range c.Metrics {
 		if m.Name == metric {