copies: mark checkcopies as internal with the _ prefix

copies: split u1/u2 to u1u/u2u and u1r/u2r These will be made different in case of grafts by another patch in this series.

copies: style fixes and add comment

copies: limit is an optimization, and doesn't provide guarantees

revset: do not rewrite ':y' to '0:y' (issue5385) That's no longer valid since the revision 0 may be hidden. Bypass validating the existence of '0' and filter it by spanset.

revset: extract function that creates range set from computed revisions So we can pass m=0 to _makerangeset() even if the revision 0 is hidden. Hidden revisions are filtered by spanset.