Add an override for the base branch

Author: peter-evans

README.md

@@ -51,6 +51,7 @@ These variables are *all optional*. If not set, sensible default values will be
 | `PULL_REQUEST_TEAM_REVIEWERS` | A comma separated list of GitHub teams to request a review from. | none |
 | `PULL_REQUEST_MILESTONE` | The number of the milestone to associate this pull request with. | none |
 | `PULL_REQUEST_BRANCH` | The branch name. See **Branch naming** below for details. | `create-pull-request/patch` |
+| `PULL_REQUEST_BASE` | Overrides the base branch. **Use with caution!** | Defaults to the currently checked out branch. |
 | `BRANCH_SUFFIX` | The branch suffix type. Valid values are `short-commit-hash`, `timestamp`, `random` and `none`. See **Branch naming** below for details. | `short-commit-hash` |
 
 **Output environment variables**

create-pull-request.py

@@ -184,12 +184,17 @@ def process_event(event_name, event_data, repo, branch, base, remote_exists):
     # Set the base branch
     github_ref = os.environ['GITHUB_REF']
     if github_ref.startswith('refs/pull/'):
+        # Switch to the merging branch instead of the merge commit
         base = os.environ['GITHUB_HEAD_REF']
-        # Reset to the merging branch instead of the merge commit
-        repo.git.checkout(base)
     else:
         base = github_ref[11:]
 
+    # Optional base override
+    base = os.getenv('PULL_REQUEST_BASE', base)
+
+    # Checkout the base branch
+    repo.git.checkout(base)
+
     # Skip if the current branch is a PR branch created by this action
     if base.startswith(branch):
         print("Branch '%s' was created by this action. Skipping." % base)