dashboard: fix perf graph view

1. Add missing comma to empty records after addition of Ann column.
2. Fix off-by-one in commit range calculation.
Release tags refer to the last commit of the release,
so there is no need to subtract 1 from them.

TBR=adg
R=adg, bradfitz
CC=golang-codereviews
https://golang.org/cl/157120043

dashboard/app/build/perf_graph.go

@@ -71,7 +71,7 @@ func perfGraphHandler(w http.ResponseWriter, r *http.Request) {
 			logErr(w, r, err)
 			return
 		}
-		startCommitNum = comFrom.Num - 1
+		startCommitNum = comFrom.Num
 
 	retry:
 		if commitTo == "tip" {
@@ -87,7 +87,7 @@ func perfGraphHandler(w http.ResponseWriter, r *http.Request) {
 				logErr(w, r, err)
 				return
 			}
-			endCommitNum = comTo.Num
+			endCommitNum = comTo.Num + 1
 		}
 		if endCommitNum <= startCommitNum {
 			// User probably selected from:go1.3 to:go1.2. Fix go1.2 to tip.

dashboard/app/build/perf_graph.html

@@ -29,7 +29,7 @@
               {{if .Val}}
                 {{.Val}}, {{.Certainty}}, '{{.Hint}}', '{{.Ann}}',
               {{else}}
-                ,,,
+                ,,,,
               {{end}}
             {{end}}
           ],