Added a bot-generated PR-comment with a simple status table

The littlefs CI is actually in a nice state that generates a lot of
information about PRs (code/stack/struct changes, line/branch coverage
changes, benchmark changes), but GitHub's UI has changed overtime to
make CI statuses harder to find for some reason.

This bot comment should hopefully make this information easy to find
without creating too much noise in the discussion. If not, this can
always be changed later.

.github/workflows/status.yml

@@ -4,11 +4,15 @@ on:
     workflows: [test]
     types: [completed]
 
+defaults:
+  run:
+    shell: bash -euv -o pipefail {0}
+
 jobs:
+  # forward custom statuses
   status:
     runs-on: ubuntu-22.04
     steps:
-      # custom statuses?
       - uses: dawidd6/action-download-artifact@v2
         continue-on-error: true
         with:
@@ -53,3 +57,44 @@ jobs:
                 target_url: env.TARGET_URL,
               }' | tee /dev/stderr)"
           done
+
+  # forward custom pr-comments
+  comment:
+    runs-on: ubuntu-22.04
+
+    # only run on success (we don't want garbage comments!)
+    if: ${{github.event.workflow_run.conclusion == 'success'}}
+
+    steps:
+      # generated comment?
+      - uses: dawidd6/action-download-artifact@v2
+        continue-on-error: true
+        with:
+          workflow: ${{github.event.workflow_run.name}}
+          run_id: ${{github.event.workflow_run.id}}
+          name: comment
+          path: comment
+      - name: update-comment
+        continue-on-error: true
+        run: |
+          ls comment
+          for s in $(shopt -s nullglob ; echo comment/*.json)
+          do
+            export NUMBER="$(jq -er '.number' $s)"
+            export BODY="$(jq -er '.body' $s)"
+
+            # check that the comment was from the most recent commit on the
+            # pull request
+            [ "$(curl -sS -H "authorization: token ${{secrets.BOT_TOKEN}}" \
+                "$GITHUB_API_URL/repos/$GITHUB_REPOSITORY/pulls/$NUMBER" \
+                | jq -er '.head.sha')" \
+              == ${{github.event.workflow_run.head_sha}} ] || continue
+
+            # update comment
+            curl -sS -X POST -H "authorization: token ${{secrets.BOT_TOKEN}}" \
+              "$GITHUB_API_URL/repos/$GITHUB_REPOSITORY/issues/`
+                `$NUMBER/comments" \
+              -d "$(jq -n '{
+                body: env.BODY,
+              }' | tee /dev/stderr)"
+          done