rebase: handle the case when nothing to rebase (dry-run) In dry-run mode, even when there was no rebase in progress it tried to abort (for context, at the end of dry-run mode we abort the in-memory performed rebase) because we were not early checking if there was any rebase started. Changes in test file reflect the fixed behavior. Differential Revision: https://phab.mercurial-scm.org/D9659