dashboard: use correct hg template for parents

The 'parents' field is documented to be blank if there's only one
simple parent. We want 'p1node', which is the hex commit of the
first parent.

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/142530043

dashboard/builder/vcs.go

@@ -202,7 +202,7 @@ type HgLog struct {
 const xmlLogTemplate = `
         <Log>
         <Hash>{node|escape}</Hash>
-        <Parent>{parents}</Parent>
+        <Parent>{p1node}</Parent>
         <Author>{author|escape}</Author>
         <Date>{date|rfc3339date}</Date>
         <Desc>{desc|escape}</Desc>