chore: simplify an example of diff checking (#69)

The original sample hides the result of `git diff`. So if `git diff` returns an
error(e.g. invalid file name specified), the result will be success.

We can use `--exit-code ` option if we only want to know whether differences
exist without depending on other commands.
Ref: https://git-scm.com/docs/git-diff#Documentation/git-diff.txt---exit-code

.github/workflows/check-dist.yml

@@ -38,7 +38,7 @@ jobs:
 
       - name: Compare the expected and actual dist/ directories
         run: |
-          if [ "$(git diff --ignore-space-at-eol dist/ | wc -l)" -gt "0" ]; then
+          if ! git diff --ignore-space-at-eol --exit-code --quiet dist/; then
             echo "Detected uncommitted changes after build.  See status below:"
             git diff
             exit 1