CI: Make the notes-push.yml workflow fetch the entire repo history

This change makes the notes-push.yml·workflow fetch the entire history
on each push, rather than just the one HEAD commit it otherwise sees.

Because we push multiple commits from PR merges, git-gloss need access
on each push to an arbitrary number of commits (however many commits
were pushed in a PR that got merged). There’s no easy way from GH
Actions to know how may commits got pushed at the same time. So we
instead fetch the whole history.

.github/workflows/notes-push.yml

@@ -9,7 +9,9 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
       - uses: fregante/setup-git-user@v2
       - run: |
           git fetch origin "refs/notes/*:refs/notes/*"