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.

registrar: add delayregistrar class to register function in extensions 'delayregistrar' delays actual registration of function until 'setup()' invocation on it.