rebase: avoid clobbering wdir() with --dry-run or --confirm (issue6291)
See 56d3e0b499df for the source of adding originalwd to the list of things that
cause wdir to be updated. That change didn't come with tests, and attempts to
recreate the scenario described have thus far failed.
Differential Revision: https://phab.mercurial-scm.org/D8489