merge: simplify backwards revert test, ready to combine with other cases

merge: allow merging going backwards New behavior is generally superior and more correct, except possibly with regards to missing files. hg up . is now effectively a no-op, which is probably the desired behavior for people expecting to move to tip, but may surprise people who were expecting deleted files to reappear. case 1: update to . a-w -> a-w classic: ancestor a missing recreated right? rmed recreated WRONG added forgotten WRONG changed preserved RIGHT conflicted can't happen backward merge: ancestor a (NO EFFECT) missing missing wrong? rm'ed rm'ed RIGHT added preserved RIGHT changed preserved RIGHT conflicted can't happen case 2: update to ancestor of . a-b-w -> b-w \ a classic: ancestor a missing recreated right? rmed recreated wrong? added forgotten wrong? changed preserved RIGHT conflicted preserved wrong? backwards merge: ancestor b missing missing or conflict right? rm'ed missing or conflict right? changed preserved RIGHT conflicted merge RIGHT added preserved right?

merge: reorder remote creation tests

merge: drop recreating case

merge: drop an overwrite test

merge: simplify a delete case

merge: reorder get cases for future simplification

merge: fix prompt keep

merge: remove a flags case

merge: pretend ancestor is local to simplify backwards and overwrite