histedit: add description about basic histedit function to command help Before this patch, there is no explicit description that histedit edits changesets between specified ancestor and the parent of the working directory: users may notice it by error message "REV is not an ancestor of working directory". This patch adds description about basic histedit function to command help of it. This patch uses term "ancestor" instead of "parent", because it seems to be more suitable, and almost all (error) messages already use it.