Add two more GitHub Actions workflows

This change adds:

* `tag.yml`: Creates a new tag every time the master or a release branch
  is pushed.
* `backport.yml`: Creates a cherry-pick in older release branches to
  keep them up to date with little cost.

Author: lhchavez

.github/workflows/backport.yml

@@ -0,0 +1,37 @@
+name: Backport to older releases
+on:
+  push:
+    branches:
+    - master
+
+jobs:
+
+  backport:
+    strategy:
+      fail-fast: false
+      matrix:
+        branch: [ 'release-0.28', 'release-0.27' ]
+    name: Backport change to branch ${{ matrix.branch }}
+
+    runs-on: ubuntu-20.04
+
+    steps:
+    - name: Check out code
+      uses: actions/checkout@v1
+      with:
+        fetch-depth: 0
+    - name: Create a cherry-pick PR
+      run: |
+        if ! git diff --quiet HEAD:vendor/libgit2 HEAD^:vendor/libgit2; then
+          echo '::warning::Skipping cherry-pick since it is a vendored libgit2 bump'
+          exit 0
+        fi
+
+        git checkout ${{ matrix.branch }}
+        git switch -c "cherry-pick-${{ github.id }}-${{ matrix.branch }}"
+        git cherry-pick -x ${{ github.sha }}
+        gh pr create \
+          --base ${{ matrix.branch }} \
+          --title "$(git show --format="%s" --no-patch HEAD)" \
+          --body "$(git show --format="%b" --no-patch HEAD)"
+

.github/workflows/tag.yml

@@ -0,0 +1,28 @@
+name: Tag new releases
+on:
+  push:
+    branches:
+    - master
+    - release-*
+
+jobs:
+
+  tag-release:
+    name: Bump tag in ${{ github.ref }}
+
+    runs-on: ubuntu-20.04
+
+    steps:
+    - name: Check out code
+      uses: actions/checkout@v1
+      with:
+        fetch-depth: 0
+    - name: Bump version and push tag
+      id: bump-version
+      uses: anothrNick/github-tag-action@c170e78287f338a4af0dc49e033e50e5a072d82b
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        WITH_V: true
+        DEFAULT_BUMP: patch
+        TAG_CONTEXT: branch
+        RELEASE_BRANCHES: master,release-.*