Merge branch 'rr/rebase-autostash'

* rr/rebase-autostash: rebase: implement --[no-]autostash and rebase.autostash rebase --merge: return control to caller, for housekeeping rebase -i: return control to caller, for housekeeping am: return control to caller, for housekeeping rebase: prepare to do generic housekeeping rebase -i: don't error out if $state_dir already exists am: tighten a conditional that checks for $dotest