[MERGE] Add cross references to bzr help.

[Robert Collins]

This adds programmatic cross references to 'bzr help'. I've updated the
built in commands, but plugins may want to use it too. For instance,
'bzr gtk' probably wants to add 'gcommit' to the _see_also list for 'bzr
commit'. I wasn't sure where to write that up though :(.

-Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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