chore: when diffing, update existing comments (#1149)

LW-10047

Author: michalrus

.github/workflows/std.yml

@@ -218,9 +218,31 @@ jobs:
       - name: Post Comment on the PR
         env:
           GH_TOKEN: ${{ github.token }}
+          COMMENT_MARKER: "nix-helm-diff-777f3796-c80d-4d68-bf4f-8faad564f03f"
         run: |
           prNumber=$(cut -d/ -f1 <<<'${{ github.ref_name }}')
-          gh pr comment "$prNumber" --body "$(cat pr-comment.md)"
+          # The `gh` command doesn’t return numeric comment ids, but instead node_ids, which don’t work with the regular API
+          # Why the regular API? Because the `gh` command doesn’t support editing comments:
+          existingCommentId=$(
+            curl --fail-with-body -sSL \
+              -X GET \
+              -H "Authorization: Bearer $GH_TOKEN" \
+              -H "Accept: application/vnd.github+json" \
+              -H "X-GitHub-Api-Version: 2022-11-28" \
+              "https://api.github.com/repos/${GITHUB_REPOSITORY}/issues/${prNumber}/comments" \
+            | jq -r --arg commentMarker "$COMMENT_MARKER" 'first(.[] | select(.body | contains($commentMarker)) | .id)'
+          )
+          if [ -z "$existingCommentId" ]; then
+            gh pr comment "$prNumber" --body "$(cat pr-comment.md) <!-- $COMMENT_MARKER -->"
+          else
+            curl --fail-with-body -sSL \
+              -X PATCH \
+              -H "Authorization: Bearer $GH_TOKEN" \
+              -H "Accept: application/vnd.github+json" \
+              -H "X-GitHub-Api-Version: 2022-11-28" \
+              "https://api.github.com/repos/${GITHUB_REPOSITORY}/issues/comments/${existingCommentId}" \
+              -d "$(jq --null-input --arg body "$(cat pr-comment.md) <!-- $COMMENT_MARKER -->" '{body: $body}')"
+          fi
 
 
   deploy: