Move rubocop check to pre_commit hook

Running rubocop checks on push is frustrating, as correcting any errors
requires either a new commit or an --amend. By moving rubocop checks to
pre_commit, we can instead prevent issues it complains about from
entering the tree entirely, helping users to prevent stubbing their toe
when they later attempt to push.

lefthook.yml

@@ -41,11 +41,6 @@ pre-push:
       files: git diff --name-only --diff-filter=d $(git merge-base origin/master HEAD)..HEAD
       glob: '*.{js,vue,graphql}'
       run: yarn run prettier --check {files}
-    rubocop:
-      tags: backend style
-      files: git diff --name-only --diff-filter=d $(git merge-base origin/master HEAD)..HEAD
-      glob: '*.{rb,rake}'
-      run: REVEAL_RUBOCOP_TODO=0 bundle exec rubocop --parallel --force-exclusion {files}
     sidekiq-queues:
       tags: backend
       files: git diff --name-only --diff-filter=d $(git merge-base origin/master HEAD)..HEAD
@@ -115,6 +110,11 @@ pre-push:
 pre-commit:
   parallel: true
   commands:
+    rubocop:
+      tags: backend style
+      files: git diff --name-only --diff-filter=d --staged
+      glob: '*.{rb,rake}'
+      run: REVEAL_RUBOCOP_TODO=0 bundle exec rubocop --parallel --force-exclusion {files}
     secrets-detection:
       tags: secrets
       files: git diff --name-only --diff-filter=d --staged