ci(merge): add fast-forward merge action

comment-out with `/fast-forward` to rebase branch into target branch

Signed-off-by: Cyril Fougeray <[email protected]>

.github/workflows/fast-forward.yml

@@ -0,0 +1,28 @@
+name: fast-forward
+on:
+  issue_comment:
+    types: [created, edited]
+jobs:
+  fast-forward:
+    # Only run if the comment contains the /fast-forward command.
+    if: ${{ contains(github.event.comment.body, '/fast-forward')
+      && github.event.issue.pull_request }}
+    runs-on: ubuntu-latest
+    env:
+      GITHUB_TOKEN: ${{ secrets.GIT_HUB_TOKEN }}
+
+    permissions:
+      contents: write
+      pull-requests: write
+      issues: write
+
+    steps:
+      - name: Fast forwarding
+        uses: sequoia-pgp/fast-forward@v1
+        with:
+          merge: true
+          # To reduce the workflow's verbosity, use 'on-error'
+          # to only post a comment when an error occurs, or 'never' to
+          # never post a comment.  (In all cases the information is
+          # still available in the step's summary.)
+          comment: on-error

.github/workflows/pull-request-ff.yml

@@ -0,0 +1,28 @@
+# Helper for Fast Forward Merge action
+#
+# - checks if the PR can be fast-forwarded.
+name: pull-request
+on:
+  pull_request:
+    types: [opened, reopened, synchronize]
+jobs:
+  check-fast-forward:
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: read
+      # We appear to need write permission for both pull-requests and
+      # issues in order to post a comment to a pull request.
+      pull-requests: write
+      issues: write
+
+    steps:
+      - name: Checking if fast forwarding is possible
+        uses: sequoia-pgp/fast-forward@v1
+        with:
+          merge: false
+          # To reduce the workflow's verbosity, use 'on-error'
+          # to only post a comment when an error occurs, or 'never' to
+          # never post a comment.  (In all cases the information is
+          # still available in the step's summary.)
+          comment: never