config: propose some sample global config file

An example of what could be suggested to the user as a global config
file. Trying to be conservative here, and only suggesting the safest
possible extensions. In addition to the user-level extensions, the
blackbox extension is something a sysadmin might reasonable want to
enable for every repo on the system.

mercurial/config.py

@@ -31,7 +31,15 b' username ='
 
     'global':
 """# example system-wide hg config (see "hg help config" for more info)
-""",
+
+[extensions]
+# Uncomment these lines for some possible extensions
+# (see "hg help extensions" for more info)
+#
+# blackbox =
+# progress =
+# color =
+# pager =""",
 }
 
 class config(object):