Template workflow: smarter syncing with the styles repo

Current syncing procedure that used in the Template workflow fails for:

1. Lessons that are, in fact, nsync with the styles repo.
2. For lessons that use The Carpentries' remote theme and have deleted
   some of the files.

This PR makes this step a little bit more intelligent and takes into
account the above two scenarios.

Author: maxim-belkin

.github/workflows/template.yml

@@ -15,7 +15,6 @@ jobs:
         lesson: [swcarpentry/shell-novice, datacarpentry/r-intro-geospatial, librarycarpentry/lc-git]
         os: [ubuntu-20.04, macos-latest, windows-latest]
         experimental: [false]
-        remote-theme: [false]
         include:
           - os: ubuntu-20.04
             os-name: Linux
@@ -34,7 +33,6 @@ jobs:
             experimental: true
             os: ubuntu-20.04
             os-name: Linux
-            remote-theme: true
           - lesson: carpentries/lesson-example
             lesson-name: (CP) Lesson Example
             experimental: false
@@ -76,26 +74,46 @@ jobs:
           path: lesson
           fetch-depth: 0
 
-      - name: Determine the proper reference to use
-        id: styles-ref
+      - name: Sync lesson with carpentries/styles
+        working-directory: lesson
         run: |
-          if [[ -n "${{ github.event.pull_request.number }}" ]]; then
-            echo "::set-output name=ref::refs/pull/${{ github.event.pull_request.number }}/head"
+          if [[ -n "${{ github.event.pull_request.number }}" ]]
+          then
+            ref="refs/pull/${{ github.event.pull_request.number }}/head"
           else
-            echo "::set-output name=ref::gh-pages"
+            ref="gh-pages"
           fi
 
-      - name: Sync lesson with carpentries/styles
-        if: ${{ ! matrix.remote-theme }}
-        working-directory: lesson
-        run: |
           git config --global user.email "[email protected]"
           git config --global user.name "The Carpentries Bot"
+
           git remote add styles https://github.com/carpentries/styles.git
-          git config --local remote.styles.tagOpt --no-tags
-          git fetch styles ${{ steps.styles-ref.outputs.ref }}:styles-ref
-          git merge -s recursive -Xtheirs --no-commit styles-ref
-          git commit -m "Sync lesson with carpentries/styles"
+          git fetch styles $ref:styles-ref
+
+          # Sync up only if necessary
+          if [[ $(git rev-list --count HEAD..styles-ref) != 0 ]]
+          then
+
+            # The merge command below might fail for lessons that use remote theme
+            # https://github.com/carpentries/carpentries-theme
+            if ! git merge -s recursive -Xtheirs --no-commit styles-ref
+            then
+
+              # Remove "deleted by us, unmerged" files from the staging area.
+              # these are the files that were removed from the lesson
+              # but are still present in the carpentries/styles repo
+              git rm $(git diff --name-only --diff-filter=DU)
+
+              # If there are still "unmerged" files,
+              # let's raise an error and look into this more closely
+              if [[ -n $(git diff --name-only --diff-filter=U) ]]
+              then
+                exit 1
+              fi
+            fi
+
+            git commit -m "Sync lesson with carpentries/styles"
+          fi
 
       - name: Look for R-markdown files
         id: check-rmd