help: only call doc() when it is callable

`hg help -k` on my machine was aborting because the hg-prompt extension
was inserting a string and not a function into help.helptable and help
was blindly calling it.

This patch changes keyword searching to be more robust against
unexpected types.

mercurial/help.py

@@ -86,9 +86,10 b' def topicmatch(kw):'
                'extensioncommands': [],
                }
     for names, header, doc in helptable:
+        # Old extensions may use a str as doc.
         if (sum(map(lowercontains, names))
             or lowercontains(header)
-            or lowercontains(doc())):
+            or (callable(doc) and lowercontains(doc()))):
             results['topics'].append((names[0], header))
     import commands # avoid cycle
     for cmd, entry in commands.table.iteritems():