How to stop bash editing the history when I reuse and modify an entry?

Turns out revert-all-at-newline is the answer. I needed to include set revert-all-at-newline on in my ~/.inputrc file, since using the set command at the bash prompt had no effect. (Then, of course, I had to start a new shell.)

Also, I found that ~/.inputrc is loaded instead of /etc/inputrc if present, which means that any defaults defined in the latter are no longer active when you create ~/.inputrc. To fix this, start ~/.inputrc with $include /etc/inputrc.

Thanks to @StéphaneChazelas for pointing me in the right direction.