cat: support cat with explicit paths in subrepos The cat command with an explicit path into a subrepo is now handled by invoking cat on the file, from that subrepo. The previous behavior was to complain that the file didn't exist in the revision (of the top most repo). Now when the file is actually missing, the revision of the subrepo is named instead (though it is probably desirable to continue naming the top level repo). The documented output formatters %d and %p reflect the path from the top level repo, since the purpose of this is to give the illusion of a unified repository. Support for the undocumented (for cat) formatters %H, %R, %h, %m and %r was added long ago (I tested back as far as 0.5), but unfortunately these will reflect the subrepo node instead of the parent context. The previous implementation was a bit loose with the return value, i.e. it would return 0 if _any_ file requested was cat'd successfully. This maintains that behavior.