CMake Updater: Output difference in expectations (#3314)

Teach the script to indicate what is different on failure rather than
keeping quiet. This helps when the script is failing in CI but not
locally.

### Motivation:

The script is reporting that there are missing changes in CI on
swift-crypto, but I am unable to reproduce them locally. This is to get
immediate insight on why the check fails rather than keeping quiet.

### Modifications:

Include the diff in the update-cmake-lists script.

### Result:

update-cmake-lists script outputs the difference between what is in a PR
vs what it expects to see instead of shrugging and leaving it as an
exercise to the user.

Author: etcwilde

scripts/update-cmake-lists.sh

@@ -129,7 +129,7 @@ done
 
 if [[ "${fail_on_changes}" == true ]]; then
     if [ -n "$(git status --untracked-files=no --porcelain)" ]; then
-        fatal "Changes in the cmake files detected. Please update."
+        fatal "Changes in the cmake files detected. Please update. -- $(git diff)"
     else
         log "✅ CMake files are up-to-date."
     fi