samplehgrcs: update the list of suggested extensions

Back in the day, this was color and pager, both of which are now
default. Churn isn't that popular, but the other four below
(obviously?) are.

mercurial/ui.py

@@ -100,7 +100,10 b' username ='
 # uncomment the lines below to enable some popular extensions
 # (see 'hg help extensions' for more info)
 #
-# churn =
+# histedit =
+# rebase =
+# shelve =
+# uncommit =
 """,
 
     'cloned':