Prevent actions duplication on noop merge commits

Author: dtolnay

.github/workflows/ci.yml

@@ -12,8 +12,13 @@ env:
   RUSTFLAGS: -Dwarnings
 
 jobs:
+  pre_ci:
+    uses: dtolnay/.github/.github/workflows/pre_ci.yml@master
+
   test:
     name: Rust ${{matrix.rust}}
+    needs: pre_ci
+    if: needs.pre_ci.outputs.continue
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
@@ -29,6 +34,8 @@ jobs:
 
   doc:
     name: Documentation
+    needs: pre_ci
+    if: needs.pre_ci.outputs.continue
     runs-on: ubuntu-latest
     env:
       RUSTDOCFLAGS: -Dwarnings