Make autointegrate action idempotent (#34)

This allows us to trigger it on other events (e.g. a cron) to update
for new LLVM commits without constantly clobbering the auto-integrate
branch.

Also switches to a personal access token (for a bot) so that there
aren't issues pushing workflow files (which I ran into while testing)
and so that the action can trigger webhooks for other automation (e.g.
builds).

I'm experiencing some intermittent failures that look like
actions/checkout#363, but those were
happening before this change as well.

Tested:
Ran on my fork:
https://github.com/GMNGeoffrey/llvm-bazel/runs/1201208655
Second run:
https://github.com/GMNGeoffrey/llvm-bazel/runs/1201226878

Author: GMNGeoffrey

.github/workflows/auto_integrate.yml

@@ -27,19 +27,26 @@ jobs:
           fetch-depth: 0
           submodules: true
           ref: ${{ env.MERGING_BRANCH }}
+          token: ${{ secrets.WRITE_ACCESS_TOKEN }}
       - name: Fetching integration branch
         run: |
           git fetch --no-tags --prune origin "${INTEGRATION_BRANCH?}:${INTEGRATION_BRANCH?}"
       - name: Setting git config
         run: |
-          git config --local user.email "llvm-bazel-github-actions-bot@google.com"
+          git config --local user.email "llvm-bazel-github-bot@google.com"
           git config --local user.name "Auto Integrate Action"
       - name: Merging in changes
         run: |
-          ./scripts/clobber_merge.sh "${INTEGRATION_BRANCH?}" "${MERGING_BRANCH?}"
+          if ! git merge-base --is-ancestor "${MERGING_BRANCH?}" "${INTEGRATION_BRANCH?}"; then
+            ./scripts/clobber_merge.sh "${INTEGRATION_BRANCH?}" "${MERGING_BRANCH?}"
+          else
+            echo "${INTEGRATION_BRANCH?} already branches from ${MERGING_BRANCH?}"
+          fi
+          git checkout "${INTEGRATION_BRANCH?}"
           git submodule update
       - name: Running auto integration
         run: ./scripts/auto_integrate.sh
       - name: Pushing changes
         run: |
-          git push origin "${INTEGRATION_BRANCH?}" --force --tags
+          git push origin "${INTEGRATION_BRANCH?}"
+          git push origin --force --tags

scripts/traverse_llvm_revs.sh

@@ -17,6 +17,8 @@ SUBMODULE_DIR="third_party/llvm-project"
 
 pushd "${SUBMODULE_DIR?}"
 START="$(git rev-parse HEAD)"
+# For help debugging https://github.com/actions/checkout/issues/363
+git remote -v
 git checkout "${BRANCH?}"
 git pull --ff-only origin "${BRANCH?}"
 git checkout "${START?}"