fileset: add class to host special handling of initial subset
Future patches will add a function to switch mctx.ctx object so that we can
forcibly evaluate a fileset expression in a specified revision. For example,
new "revs()" function will be used to match predicate agains another revision
$ hg revert 'set:revs(42, added())'
fullmatchctx class is similar to revset.fullreposet. It will allow us to
recalculate the subset only if it is not filtered yet.
# This config file points Mercurial at the system-wide certificate
# store from the ca-certificates package.
[web]
cacerts = /etc/ssl/certs/ca-certificates.crt