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.
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