Ensure exit code on git diff can cause failure as well

.ci/check-pr-no-readme.sh

@@ -4,7 +4,8 @@ set -Eeuo pipefail
 cd "$(dirname "$(readlink -f "$BASH_SOURCE")")/.."
 
 git fetch -q https://github.com/docker-library/docs.git master
-if [ -n "$(git diff --numstat FETCH_HEAD...HEAD -- '*/README.md')" ]; then
+numstat="$(git diff --numstat FETCH_HEAD...HEAD -- '*/README.md')"
+if [ -n "$numstat" ]; then
 	echo >&2 'Error: at least one repo README.md has changed'
 	echo >&2 'These files are autogenerated, so it is unnecessary to modify them'
 	echo >&2 'Please update content.md and docker-library-bot will take care of README.md'

.github/workflows/ci.yml

@@ -39,5 +39,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
       - run: .ci/check-pr-no-readme.sh
     if: ${{ github.event_name == 'pull_request' }}