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revset: use delayregistrar to register predicate in extension easily

Previous patch introduced 'revset.predicate' decorator to register
revset predicate function easily.

But it shouldn't be used in extension directly, because it registers
specified function immediately. Registration itself can't be restored,
even if extension loading fails after that.

Therefore, registration should be delayed until 'uisetup()' or so.

This patch uses 'extpredicate' decorator derived from 'delayregistrar'
to register predicate in extension easily.

This patch also tests whether 'registrar.delayregistrar' avoids
function registration if 'setup()' isn't invoked on it, because
'extpredicate' is the first user of it.

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