transaction: the base of a new test file checking transaction abort issue See inline documentation for details. See other changesets for actual cases.

setup: support building from an ongoing merge Before this change the two parents from the merge would duplicate some command output and modify some others in a way python 3.11 chokes on.

rhg: in path_encode, simplify a bit more Use the slices for `basename` and `ext` instead of dealing with offsets.