Output a summary and not an annotation

An original implementation of this used annotations, rather job
summaries but these don't really work that well.

We can't comment on files which have changed outside of the diff, so the
output is mostly shown in the log output or the autogenerated job
summary. Instead, we assemble a custom output which is much nicer.

https://github.blog/2022-05-09-supercharging-github-actions-with-job-summaries/
https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#adding-a-job-summary

bin/diff-check

@@ -2,11 +2,11 @@
 
 set -e
 
-message="has changed"
-
 if ! git diff-index --quiet HEAD; then
+	printf 'These files changed when running the command:\n\n'
+
 	git diff --name-only | while read -r n ; do
-		echo "::error file=$n::$n $message"
+		echo "* $n"
 	done
 
 	exit 1

spec/black_box/diff-check_spec.rb

@@ -17,13 +17,17 @@
   end
 
   context "with a file change" do
-    it "emits a annotation with the file" do
+    it "emits a summary with the files changed" do
       session = create_session
 
       session.run("echo 'new text' >> README")
 
-      expect(session.run("diff-check")).
-        to have_stdout("::error file=README::README has changed\n")
+      expect(session.run("diff-check")).to have_stdout(<<~SUMMARY
+        These files changed when running the command:
+
+        * README
+      SUMMARY
+      )
     end
 
     it "exits with a status of 1" do