follow up fix to c51114a1.

This still is slightly more robust than the original in the sense that it's not affected by the presence/absence of the trailing '/' in itUrl.

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Comment from @ohtake:

getContextPath() will return "" if Jenkins is served at ROOT.

Assume that getContextPath() => "", itUrl => "iturl" and urlName => "urlName",
value returned was "/iturl/urlName", but is "iturl/urlName" (no slash at 0).

core/src/main/java/hudson/Functions.java

@@ -1204,7 +1204,7 @@ public class Functions {
             return joinPath(Stapler.getCurrentRequest().getContextPath(),urlName);
         else
             // relative URL name
-            return joinPath(Stapler.getCurrentRequest().getContextPath(),itUrl,urlName);
+            return joinPath(Stapler.getCurrentRequest().getContextPath()+'/'+itUrl,urlName);
     }
 
     /**