Sync src/ with date in PO file when publishing

Before, we always used the latest English source files when
publishing. This means that translations become outdated as soon as
anything is changed in the source.

This PR changes will instead freeze translations in place: they will
keep using the same English source files until a new POT file is
merged into the translation. We do this by relying on the
POT-Creation-Date field in the PO files.

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

@@ -1,15 +1,16 @@
 name: Publish
 
 on:
-  push:
-    branches:
-      - main
-  workflow_dispatch:
+  pull_request:
+#  push:
+#    branches:
+#      - main
+#  workflow_dispatch:
 
-permissions:
-  contents: read
-  pages: write
-  id-token: write
+#permissions:
+#  contents: read
+#  pages: write
+#  id-token: write
 
 # Allow one concurrent deployment
 concurrency:
@@ -23,9 +24,9 @@ env:
 
 jobs:
   publish:
-    environment:
-      name: github-pages
-      url: ${{ steps.deployment.outputs.page_url }}
+#    environment:
+#      name: github-pages
+#      url: ${{ steps.deployment.outputs.page_url }}
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
@@ -45,8 +46,15 @@ jobs:
 
       - name: Build all translations
         run: |
+          set -x
+
           for po_lang in ${{ env.LANGUAGES }}; do
               echo "::group::Building $po_lang translation"
+              POT_CREATION_DATE=$(grep --max-count 1 '^"POT-Creation-Date:' po/$po_lang.po | 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 +63,14 @@ 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