hgweb: rename 'currentbaseline' template keyword to 'basenode' stable

hgweb: rename 'currentbaseline' template keyword to 'basenode'

Shorter and clearer. This keyword represents the node we're currently
diffing against.

--- a/mercurial/hgweb/webcommands.py	Fri Feb 01 09:58:25 2013 -0600
+++ b/mercurial/hgweb/webcommands.py	Fri Feb 01 10:12:41 2013 -0600
@@ -293,7 +293,7 @@
                 node=ctx.hex(),
                 parent=webutil.parents(ctx),
                 child=webutil.children(ctx),
-                currentbaseline=basectx.hex(),
+                basenode=basectx.hex(),
                 changesettag=showtags,
                 changesetbookmark=showbookmarks,
                 changesetbranch=showbranch,