Add diff test

Author: Manishearth

.github/workflows/rust.yml

@@ -119,6 +119,8 @@ jobs:
         run: cargo build -p temporal_capi
       - name: Regen
         run: cargo run -p diplomat-gen
+      - name: Check diff
+        run: git diff --exit-code
       # Todo: eventually we should check in bindings and test them
       - name: Makefile tests
         run: cd temporal_capi/cpp_tests && make