Remove guide comments action (#70739)

Removes all default comments after a PR is opened (and only when it is opened).

Means that I can turn on the pr body merge option without keeping all the cruft

.github/workflows/remove_guide_comments.yml

@@ -0,0 +1,18 @@
+# Removes guide comments from PRs when opened, so that when we merge them
+# and reuse the pull request description, the clutter is not left behind
+name: Remove guide comments
+on:
+  pull_request:
+    types: [opened]
+jobs:
+  remove_guide_comments:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v3
+    - name: Remove guide comments
+      uses: actions/github-script@v6
+      with:
+        script: |
+          const { removeGuideComments } = await import('${{ github.workspace }}/tools/pull_request_hooks/removeGuideComments.js')
+          await removeGuideComments({ github, context })

tools/pull_request_hooks/removeGuideComments.js

@@ -0,0 +1,36 @@
+import fs from "fs";
+
+const REGEX_COMMENT = /<!--.+?-->/g;
+
+// Make sure we only remove default comments
+const comments = [];
+
+for (const match of fs
+  .readFileSync(".github/PULL_REQUEST_TEMPLATE.md", { encoding: "utf8" })
+  .matchAll(REGEX_COMMENT)) {
+  comments.push(match[0]);
+}
+
+function escapeRegex(string) {
+  return string.replace(/[-\/\\^$*+?.()|[\]{}]/g, "\\$&");
+}
+
+export async function removeGuideComments({ github, context }) {
+  let newBody = context.payload.pull_request.body;
+
+  for (const comment of comments) {
+    newBody = newBody.replace(
+      new RegExp(`^\\s*${escapeRegex(comment)}\\s*`, "gm"),
+      "\n"
+    );
+  }
+
+  if (newBody !== context.payload.pull_request.body) {
+    await github.rest.pulls.update({
+      pull_number: context.payload.pull_request.number,
+      repo: context.repo.repo,
+      owner: context.repo.owner,
+      body: newBody,
+    });
+  }
+}