mq: implement log() on dummyui Otherwise ui.log() in extensions.py would explode.

ui: install logger that sends debug.extensions messages to stderr This will replace the custom log function introduced at d58958676b3c "extensions: add detailed loading information."

blackbox: resurrect recursion guard If I added ui.log() to hg.repository() function, test-merge-subrepos.t exploded. The problem is that the blackbox may create new repository instance while logging is active, and the created repository owns its new ui derived from the baseui, not from the ui which is processing the active logging. I tried to work around the issue in ui.log(), but that turned out to be not easy. We shouldn't globally lock the ui.log() since there may be more than one active repo/ui instances in threaded environment. We could store the logging state in thread-local storage, but that seems unnecessarily complex. So this patch reintroduces the _inlog flag to per-repository logger instances.

tests: filter out uninteresting log events This helps adding more log()s without updating the tests.

context: error out if basefilectx.cmp() is called without self._filenode The base implementation can't handle such cases because the filelog has no knowledge about the working directory. Loading self._filenode should have no extra cost since self.size() would load it anyway.

context: collapse complex condition to see if filelog have to be compared It's hard to read. I'd rather make the return statement duplicated.