merge: expand environment variables and ~/ in tool.executable hgrc(5) already implies that this works, so we might as well support it. Another approach would be to implement this in util.findexe(): that would benefit other callers of findexe(), e.g. convert and anyone calling the user's editor. But findexe() is really implemented in both posix.py and windows.py, so this would make both of those modules depend on util.py: not good. So keep it narrow and only for merge tools.