dagop: put start fctx into visit dict of filectxancestors() Prepares for multiple start revisions/files.

dagop: change visit dict of filectxancestors() indexed solely by rev In future patches, a max heap will be used to compute the next revision to visit.

dagop: use fctx.rev() consistently in filectxancestors() We can't use fctx.linkrev() to sort fctxs coming from multiple files. This was changed at 24b57c3899f8 due to performance issue, but we know we evaluate parent.rev() in revset anyway.