Add optional git hook to run formatter pre-push.

.gitignore

@@ -32,3 +32,4 @@ maps.key
 /local/
 kls_database.db
 .kotlin
+lefthook-local.yml

CONTRIBUTING.md

@@ -62,7 +62,7 @@ You will need to [sign our CLA](https://signal.org/cla/) before your pull reques
 ### Follow the Code Style Guidelines
 Ensure that your code adheres to the [Code Style Guidelines](https://github.com/signalapp/Signal-Android/wiki/Code-Style-Guidelines) before submitting a pull request.
 
-You can run `./gradlew format` to automatically check conformance with some of these guidelines.
+You can run `./gradlew format` to automatically format your code. See `lefthook.yml` for instructions on how to run this as a git hook.
 
 ### Submit finished and well-tested pull requests
 Please do not submit pull requests that are still a work in progress. Pull requests should be thoroughly tested and ready to merge before they are submitted.

lefthook.yml

@@ -0,0 +1,15 @@
+# Lefthook git hooks configuration
+#
+# ========== SETUP ==========
+# 1. Install lefthook: https://lefthook.dev/installation/
+# 2. Enable hooks: lefthook install
+#
+# ========== CUSTOMIZATION ==========
+# Create lefthook-local.yml (gitignored) to override settings locally.
+# https://lefthook.dev/examples/lefthook-local.html
+
+pre-push:
+  commands:
+    format:
+      run: ./gradlew format --quiet && test -z "$(git status --porcelain)"
+      fail_text: "Formatting changes detected. Please commit them and push again."