Support shallow clones

[lubo]

Currently, a full clone is required for this action to work properly. As we need commits since the last common ancestor only, the current behavior isn't optimal on huge repositories.

So, I propose doing something like this:

1. Check out refs/pull/${{ github.event.issue.number }}/head instead of the default branch with the depth of 1. At the point of checkout, this is the only commit we know for sure we'll need and we can refer to it. Patch:

   Click to expand

   diff --git a/README.md b/README.md
   index c88b62d..a66029c 100644
   --- a/README.md
   +++ b/README.md
   @@ -24,7 +24,8 @@ jobs:
        - name: Checkout the latest code
          uses: actions/checkout@v2
          with:
   -        fetch-depth: 0
   +        fetch-depth: 1
   +        ref: refs/pull/${{ github.event.issue.number }}/head
        - name: Automatic Rebase
          uses: cirrus-actions/rebase@1.3.1
          env:
   

2. Fetch the head branch and deepen the history by the number of commits in the PR (see --deepen and --shallow-exclude fetch options). This should fetch all the remaining commits in the head branch, plus the the last common ancestor.

3. Fetch the base branch.

At this point, we should have all the commits we need to rebase the head branch.