analysis/appengine/template: truncate long label values

Change-Id: Ica888f19254759a060015e9635cffd0231721dd4
Reviewed-on: https://go-review.googlesource.com/35946Reviewed-by: Russ Cox <rsc@golang.org>

analysis/appengine/template/compare.html

@@ -59,6 +59,12 @@ table.benchstat td, table.benchstat th {
 }
 #labels .diff {
   padding-bottom: 1em;
+}
+#labels .diff table td {
+  max-width: 20em;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
 }
     </style>
   </head>