[GitHub Action] Enable periodically automatic sync for main branch from llvm-project to llvm

.github/workflows/sync-main.yml

@@ -0,0 +1,47 @@
+name: automatic sync main branch from llvm-project to llvm
+
+on:
+  schedule:
+  - cron: '*/10 * * * *'
+jobs:
+  sync-main:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          # persist-credentials: false allows us to use our own credentials for
+          # pushing to the repository.  Otherwise, the default github actions token
+          # is used.
+          persist-credentials: false
+          fetch-depth: 0
+          path: src
+      - name: Sync
+        env:
+          BRANCH: main
+          SYNC_REPO: https://github.com/llvm/llvm-project
+        run: |
+          cd $GITHUB_WORKSPACE/src
+          branch_exist=`git ls-remote --heads origin $BRANCH | wc -l`
+          if [ $branch_exist -ne 0 ]; then
+              git checkout $BRANCH
+              git pull --ff --ff-only $SYNC_REPO $BRANCH
+              if [ $? -ne 0 ]; then
+                  echo "failed to pull from $SYNC_REPO $BRANCH, abort"
+                  exit 1
+              fi
+              git_status=`git rev-list --count --left-right origin/$BRANCH...$BRANCH`
+              if [ "0	0" == "$git_status" ] ; then
+                  echo "no change, skip"
+              elif [[ "$git_status" =  0* ]] ; then
+                  git push origin
+              else
+                  echo "$BRANCH branch invalid state"
+                  exit 1
+              fi
+          else
+              git remote add upstream $SYNC_REPO
+              git fetch upstream
+              git checkout -B $BRANCH upstream/$BRANCH
+              git push origin
+          fi
+          echo "sync finished"

README.md

@@ -26,6 +26,9 @@ See [CONTRIBUTING.md](CONTRIBUTING.md) for details.
 
 ## DPC++ extensions
 
+
+
+
 DPC++ is an open, cross-architecture language built upon the ISO C++ and Khronos
 SYCL\* standards. DPC++ extends these standards with a number of extensions,
 which can be found in [sycl/doc/extensions](sycl/doc/extensions) directory.