revset: extract parsing logic out of formatspec We want to be able to perform better handling of some input when running revset (eg: `repo.revs("%ld", somerevs)`). The first step is to be able to access some of the parsed content before it gets substituted. There are many possible different substitutions, we'll add support for them gradually. In this changeset we support none, we just split some logic in a sub function as a preparatory step.