Fix check for manifest changes

The check for manifest changes was wrong on two counts. Firstly it was
supposed to check that there *are* changes, but instead it was bailing
if there were *no* changes (the opposite of what was intended). Second,
it was including changelog changes as well, when it should have only
considered manifest changes.

.circleci/scripts/release-commit-version-bump.sh

@@ -18,7 +18,7 @@ fi
 
 printf '%s\n' 'Commit the manifest version and changelog if the manifest has changed'
 
-if ! git diff --quiet CHANGELOG.md app/manifest/_base.json;
+if git diff --quiet app/manifest/_base.json;
 then
     printf '%s\n' 'No manifest changes to commit'
     exit 0