templater: introduce wrapper for smartset (API)
I want to add a template function which takes a revset as an argument:
{somefunc(..., revset(...))}
^^^^^^^^^^^
evaluates to a revslist
This wrapper will provide a method to get an underlying smartset. It should
also be good for performance since count(revset(...)) will no longer have to
fully consume the smartset for example, but that isn't the point of this
change.