[MERGE] New flag on repository format for external lookup support

[Robert Collins]

This adds a flag to repository._format indicating support for external
lookups. Nothing supports it today of course, but this lets code start
to become aware of it.

I haven't written explicit tests for the smart server client's fallback
to old servers, but I did test that by hand; I think its a small window
for failure there and AIUI Andrew is working on generic support for that
sort of thing.

I have parameterised the FindRepository tests to test the common
behaviour for both versions of the method,

-Rob
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