vfs: modernize the detection of the main thread There weren't a lot of good choices when py27 was supported, but starting with py34, `threading.main_thread()` is available. This gets us away from an undocumented, internal symbol, and drops a pytype suppression statement. It is also apparently no longer reliable after a process fork.[1][2] [1] https://stackoverflow.com/a/23207116 [2] https://github.com/python/cpython/blob/v3.6.3/Lib/threading.py#L1334

store: fix a signature mismatch for a vfs subclass This was flagged by PyCharm. I'm not sure why pytype doesn't catch this- it's not excluded from the modules that are currently checked.

lfs: fix various signature mismatches for vfs subclasses These were flagged by PyCharm. I'm not sure why pytype doesn't catch these- only `hgext/lfs/__init__.py` in the lfs extension is excluded from being checked. I'm not sure if the `*insidef` arg to `join()` was meant as an internal convencience, because I see another class that gets flagged for the same signature problem (to be fixed next). But I don't feel bold enough to make this an internal function, and provide a simplified public `join()` on the `vfs` classes. That can still be done later, if desired. For now, process the additional args and pass them along, even though there don't appear to be any current callers that provide extra args to these classes. We need all of the subclasses to agree on the signature, or they won't be considered to implement the `Vfs` protocol being developed. While we're copy/pasting from the base class, bring the type annotations along for the ride.