No need to read changelog in show(); node mandatory arg for expand()

Just re-bin the node kwarg of the hook, instead of using changelog

Same as for hardcoded branch: omit hooktype test, return w/o explicit value

Omit hook test; return w/o explicit value (==False)

No clumsy setting of node in kwfilelog.add, pass as kwarg

Expansion entirely in kwfilectx.expand()

Implement custom keyword map templates in hgrc More expensive than hardcoded keywords. But, of course, more flexible. ATM templates can only be set in hgrc, and perhaps this is enough.