[MERGE][1.6] lookup rules using get_special_file API

[Ian Clatworthy]

During the review of the rules-based preferences patch,
John and I discussed the idea of accessing the .bzrrules
file via a custom API which could provide better performance.

During my testing of the content filtering branch, it turns
out that using a custom API is actually a requirement in
order to fix an infinite recursion bug that occurs. This
patch introduces that API and uses it to find the .bzrrules
file.

The API has been designed so that it can be used for looking
up .bzrignore and other special files down the track. This
patch doesn't do that yet. I'm happy to add that now if the
reviewer feels that I should. OTOH, that change can come later
while I'd like this fix to rules in 1.6 if time permits.

Ian C.
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