Sync src/ with date in PO file when publishing

This will freeze translation in place: they will keep using the same
English Markdown source files as the starting point until a new POT
file is merged into the translation.

We still update all the files around the Markdown files: this allows
us to fix things in the theme, for example.

Part of google/mdbook-i18n-helpers#16. The
logic here should eventually be moved to somewhere in
mdbook-i18n-helpers, most likely to the renderer that @sakex is
building in google/mdbook-i18n-helpers#84.

.github/workflows/publish.yml

@@ -1,10 +1,11 @@
 name: Publish
 
 on:
-  push:
-    branches:
-      - main
-  workflow_dispatch:
+  pull_request:
+#  push:
+#    branches:
+#      - main
+#  workflow_dispatch:
 
 permissions:
   contents: read
@@ -47,6 +48,11 @@ jobs:
         run: |
           for po_lang in ${{ env.LANGUAGES }}; do
               echo "::group::Building $po_lang translation"
+              POT_CREATION_DATE=$(grep --max-count 1 '^"POT-Creation-Date:' $PO_FILE | sed -E 's/".*: (.*)\\n"/\1/')
+              if [[ $POT_CREATION_DATE == "" ]]; then
+                  POT_CREATION_DATE=now
+              fi
+              git checkout "main@{$POT_CREATION_DATE}" src
               MDBOOK_BOOK__LANGUAGE=$po_lang \
               MDBOOK_OUTPUT__HTML__SITE_URL=/comprehensive-rust/$po_lang/ \
               mdbook build -d book/$po_lang
@@ -55,14 +61,14 @@ jobs:
               echo "::endgroup::"
           done
 
-      - name: Setup Pages
-        uses: actions/configure-pages@v2
-
-      - name: Upload artifact
-        uses: actions/upload-pages-artifact@v1
-        with:
-          path: book/html
-
-      - name: Deploy to GitHub Pages
-        id: deployment
-        uses: actions/deploy-pages@v1
+#      - name: Setup Pages
+#        uses: actions/configure-pages@v2
+#
+#      - name: Upload artifact
+#        uses: actions/upload-pages-artifact@v1
+#        with:
+#          path: book/html
+#
+#      - name: Deploy to GitHub Pages
+#        id: deployment
+#        uses: actions/deploy-pages@v1