crecord: break out the help message for editing hunks This help message can be useful for other situations, such as for the review extension. It's also easier to write it at the top-level indentation with triple-quoted strings instead of inserting comment characters and newlines programmatically.

crecord: refactor hunk edit action to use ui.edit The previous version of this code did a lot of dancing around a temporary edit file that ui.edit already handles.