hgweb: avoid traceback if raw is used as default style stable

hgweb: avoid traceback if raw is used as default style

Spotted by Angel Ezquerra.

--- a/mercurial/templates/raw/map	Sat Jul 28 22:55:01 2012 +0200
+++ b/mercurial/templates/raw/map	Sun Jul 29 11:08:45 2012 -0500
@@ -1,3 +1,5 @@
+default = 'shortlog'
+shortlog = "'raw' is not a browsable style"
 mimetype = 'text/plain; charset={encoding}'
 header = ''
 footer = ''