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.