rebase: convert opts dict to bytes at once

rebase: isolate command options from internal flags I want to get rid of per-function byteskwargs(opts).

rebase: suppress warning thrown when aborting rebase in case of dryrun Before this patch dryrun output contained "rebase aborted" every time we run rebase in dryrun mode and this warning does not sound safe from a user prespective. Differential Revision: https://phab.mercurial-scm.org/D3857