Add and use autosquash action input

By using an input to trigger whether to rebase with autosquash, we can
enable other kinds of workflow configurations (e.g. other events and
criteria).

README.md

@@ -28,6 +28,8 @@ jobs:
           fetch-depth: 0 # otherwise, you will fail to push refs to dest repo
       - name: Automatic Rebase
         uses: cirrus-actions/rebase@1.4
+        with:
+          autosquash: ${{ contains(github.event.comment.body, '/rebase-autosquash') }}
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 ```

action.yml

@@ -4,6 +4,11 @@ maintainer: Cirrus Labs
 runs:
   using: 'docker'
   image: 'Dockerfile'
+inputs:
+  autosquash:
+    description: Should the rebase autosquash fixup and squash commits
+    required: false
+    default: false
 branding:
   icon: git-pull-request
   color: purple

entrypoint.sh

@@ -97,7 +97,11 @@ git fetch fork $HEAD_BRANCH
 
 # do the rebase
 git checkout -b fork/$HEAD_BRANCH fork/$HEAD_BRANCH
-git rebase origin/$BASE_BRANCH
+if [[ $INPUT_AUTOSQUASH -eq 'true' ]]; then
+	GIT_SEQUENCE_EDITOR=: git rebase -i --autosquash origin/$BASE_BRANCH
+else
+	git rebase origin/$BASE_BRANCH
+fi
 
 # push back
 git push --force-with-lease fork fork/$HEAD_BRANCH:$HEAD_BRANCH