hgweb: resurrect <span> tag on diffline to fix rendering in monoblue style stable

hgweb: resurrect <span> tag on diffline to fix rendering in monoblue style

It was removed at 606a3bf82e30 as a useless tag, but it is necessary to
apply "div.diff pre span" style.

http://selenic.com/repo/hg/rev/606a3bf82e30?style=monoblue

--- a/mercurial/templates/monoblue/map	Wed Apr 01 13:27:56 2015 -0500
+++ b/mercurial/templates/monoblue/map	Thu Apr 02 21:29:05 2015 +0900
@@ -103,7 +103,7 @@
 difflineplus = '<span class="difflineplus"><a class="linenr" href="#{lineid}" id="{lineid}">{linenumber}</a> {line|escape}</span>'
 difflineminus = '<span class="difflineminus"><a class="linenr" href="#{lineid}" id="{lineid}">{linenumber}</a> {line|escape}</span>'
 difflineat = '<span class="difflineat"><a class="linenr" href="#{lineid}" id="{lineid}">{linenumber}</a> {line|escape}</span>'
-diffline = '<a class="linenr" href="#{lineid}" id="{lineid}">{linenumber}</a> {line|escape}'
+diffline = '<span><a class="linenr" href="#{lineid}" id="{lineid}">{linenumber}</a> {line|escape}</span>'
 
 comparisonblock ='
   <tbody class="block">