If live gutter is turned on, it will run "diff" after every command

If live gutter is turned on (`git-gutter:update-interval` is non-nil), git-gutter will run `diff` after every keystroke, no matter whether it actually modified the buffer or not. Just moving the cursor causes a `diff` call. I believe https://github.com/emacsorphanage/git-gutter/issues/203 introduced this behavior.