Unlike other functions, "revs()" does not select files but switches the
evaluation context. This allow to match file with property in another revision
that the one currently evaluated.
This changeset is based on work from Yuya Nishihara.
Having sections for specific operator types assumes the user already knows what
type of operators are supported. By having a common heading, the user can
simply lookup help for "(revsets|filesets|templates).operators".
This has the nice property of visually breaking up the wall of text. It also
allows specific smaller sections to be called out. For example,
`hg help filesets.predicates` now prints just the predicate section. At the
moment, the revset headings are a superset of the fileset headings, so there is
consistency in how example, predicate and operator help is called out.
The reference to `hg help patterns` was moved to the overview section, so that
it isn't stuck in the examples section.
