Feature summary (what you would like to be able to do and where):
The "Prompt me when entering a blank edit summary (or the default undo summary)" should have a child toggle that disables the warning for redirect creations.

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
This would allow people who create a lot of redirects to use this feature without having to click publish twice to bypass the warning.

Benefits (why should this be implemented?):
Meta says that the automatic edit summary for redirect creation is adequate. Therefore, users who do not wish to write a custom summary when creating a redirect should not be warned about it. I'm proposing a separate toggle because some users may wish to never leave an automatic summary even in that case.