cmd/fix: change -diff to exit 1 if diffs exist [1.26 backport]

[gopherbot]

@dmitshur requested issue #77583 to be considered for backport to the next 1.26 minor release.

@gopherbot Please create a backport issue for Go 1.26 (original request was in #77583 (comment)).

[Rican7]

Thank you! 🙌