histedit: make comment part of the file describing rules as translatable
"i18n.gettext()" invocation for "editcomment" itself should have
little impact for whole initialization cost of histedit, because it is
already invoked for initialization of the command table entry.
'\n\n' has to be added to the text being edited separately from
"editcomment", because leading 'line feed' characters are not exported
to "hg.pot" file correctly.