hydra-update-gc-roots: Keep the most recent evaluations

We now keep all builds in the N most recent evaluations of a jobset,
rather than the N most recent builds of every job.  Note that this
means that typically fewer builds will be kept (since jobs may be
unchanged across evaluations).

src/root/edit-jobset.tt

@@ -110,7 +110,7 @@
     </div>
 
     <div class="control-group">
-      <label class="control-label">Number of builds to keep</label>
+      <label class="control-label">Number of evaluations to keep</label>
       <div class="controls">
         <input type="number" class="span3" name="keepnr" [% HTML.attributes(value => jobset.keepnr) %]></input>
       </div>