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).