chgserver: add utilities to calculate mtimehash
mtimehash is designed to detect file changes. These files include:
- single file extensions (__init__.py for complex extensions)
- mercurial/__version__.py
- python (sys.executable)
mtimehash only uses stat to check files so it's fast but not 100% accurate.
However it should be good enough for our use case.
For chgserver, once mtimehash changes, the server is considered outdated
immediately and should no longer provide service.