histedit: move outgoing processing to its own function Every piece of code extracted from the main command is a win. We simplify changeset determination in the process. Parent ceases being a list before becoming a node. We how have a root variable containing a node all the time.

histedit: move all arguments checks to the beginning of the command This changeset move all checks and raises related to arguments validation to the top of the file. This gathers all the logic in one place and clarifies the code doing actual work. This paves the way for splitting this gigantic function in separated functions. A `goal` variable is introduced in the process. It holds the action to be done by this invocation (new, continue or abort). An invalid invocation is found in the process (the new code is a bit stricter).

histedit-test: generalise --commands "-" usage This is simpler than temporary file version. There some minor test changes since commit messages are no longer modifed. There is still some tests using --commands with a real file.

histedit: allow "-" as a command file When "-" is specified as a command file, we read rules from stdin. Alter a single test to demo the behavior, but most of them could benefit from this. There is minor change in test output resulting from the fact we no longer change log messages.