debugshell: allow TortoiseHg builds to exit with the usual `quit()` command I've long been annoyed that `quit()` only randomly worked to exit the interpreter. When that happens, Ctrl+C doesn't work either (it simply prints "KeyboardInterrupt"), so then you have to `import sys` and `sys.exit()`. But it turns out that the behavior isn't random and it depended on which `hg.exe` was picked up on PATH first, because py2exe disables site initialization. I wasn't able to persuade the maintainer to allow an opt-in to initialization[1], but this works around it so that the behavior is now consistent however `hg.exe` is built. TortoiseHg 6.3.3 will be the first build that includes the site package, so handle the ImportError. [1] https://github.com/py2exe/py2exe/issues/154