Sync src/ with date in PO file when publishing (#1243)

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 This means that translations become outdated as soon as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 use up-to-date versions of all other files: this allows us to
fix things in the theme, for example. So the assumption here is that
never versions of our infrastructure remains compatible with old
versions of the Markdown files.

This approach has a problem: when files are moved, the English
translation will link to the new name. This name will not exist in the
old translations. We might be able to disable these links somehow.

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

@@ -30,6 +30,9 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v3
+        with:
+          # We need the full history below.
+          fetch-depth: 0
 
       - name: Setup Rust cache
         uses: ./.github/workflows/setup-rust-cache
@@ -46,9 +49,20 @@ jobs:
       - name: Build all translations
         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/$po_lang.po | sed -E 's/".*: (.*)\\n"/\1/')
+              if [[ $POT_CREATION_DATE == "" ]]; then
+                  POT_CREATION_DATE=now
+              fi
+              echo "::group::Building $po_lang translation as of $POT_CREATION_DATE"
+              rm -r src/
+              git restore --source "$(git rev-list -n 1 --before "$POT_CREATION_DATE" @)" src/
+
+              # Set language and adjust site URL. Clear the redirects
+              # since they are in sync with the source files, not the
+              # translation.
               MDBOOK_BOOK__LANGUAGE=$po_lang \
               MDBOOK_OUTPUT__HTML__SITE_URL=/comprehensive-rust/$po_lang/ \
+              MDBOOK_OUTPUT__HTML__REDIRECT='{}' \
               mdbook build -d book/$po_lang
               (cd book/$po_lang/exerciser && zip --recurse-paths ../html/comprehensive-rust-exercises.zip comprehensive-rust-exercises/)
               mv book/$po_lang/html book/html/$po_lang