help: avoid mutating passed-in `keep` list in `formattedhelp`

mercurial/help.py

@@ -618,6 +618,8 b' def formattedhelp(ui, name, keep=None, u'
     """
     if keep is None:
         keep = []
+    else:
+        keep = list(keep) # make a copy so we can mutate this later
     fullname = name
     section = None
     subtopic = None