[MERGE/RFC] catch-all handler for filter stack lookup

[Ian Clatworthy]

Jelmer wants to transparently map svn properties to bzr
content filters. Our current filter stack registration
function supports a dictionary mapping from

  supported value -> filter stack

but that doesn't scale to stuff like ...

  keywords = Date, Version

where the value is arbitrarily complex. This patch adds
an optional fallback handler - a callable invoked if an
unknown value is encountered. That should be good enough
I think for Jelmer to weave his magic. :-)

I'm making this an RFC because I want Jelmer to confirm
this meets his needs and because I'm not sure how best
to expand the lazy registration function Jelmer added.
Perhaps we want to register a compound object rather
than a (map,fallback) tuple? If so, we could point to that
object. Or we could point lazily to both a map and a fallback
callable?

Ian C.
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