Diff the whole push range and de-duplicate the gate-skip note

.github/workflows/test.yml

@@ -24,7 +24,7 @@ jobs:
             git fetch --no-tags origin "$GITHUB_BASE_REF"
             base="origin/$GITHUB_BASE_REF"
           else
-            base="HEAD^"
+            base='${{ github.event.before }}'
           fi
           if git diff --quiet "$base"...HEAD -- '*.go' go.mod go.sum Dockerfile .dockerignore .github data README.md; then
             echo "code=false" >> "$GITHUB_OUTPUT"

README.md

@@ -491,11 +491,10 @@ docker compose run --rm --user "$(id -u):$(id -g)" tidy  # go mod tidy
 ```
 
 Every pull request runs `docker build .` against both the latest and the lowest
-supported Go, and must pass before it can be merged. That build is the whole
+supported Go, and must pass before it can be merged — unless it touches none of
-gate — vet, complexity, format check and tests — so run it locally before pushing.
+the files the build and its tests read, in which case it's skipped (see
-A PR that touches none of the Go and module files, the Dockerfile,
+[Decisions](#decisions)). That build is the whole gate — vet, complexity, format
-`.dockerignore`, the workflow, the shipped data nor the README skips the build
+check and tests — so run it locally before pushing:
-and passes as-is (see [Decisions](#decisions)):
 
 ```sh
 docker build .                                  # latest