Use repo's default branch if no branch specified

pull-request.py

@@ -234,6 +234,22 @@ def find_pull_request(listing, source):
                 return entry
 
 
+def find_default_branch():
+    """Find default branch for a repo (only called if branch not provided)
+    """
+    response = requests.get(REPO_URL)
+
+    # Case 1: 404 might need a token
+    if response.status_code == 404:
+        response = requests.get(REPO_URL, headers=HEADERS)
+    if response.status_code != 200:
+        abort_if_fail(response, "Unable to retrieve default branch")
+
+    default_branch = response.json()["default_branch"]
+    print("Found default branch: %s" % default_branch)
+    return default_branch
+
+
 def add_reviewers(entry, reviewers, team_reviewers):
     """Given regular or team reviewers, add them to a PR.
 
@@ -343,8 +359,8 @@ def main():
     if not branch_prefix:
         print("No branch prefix is set, all branches will be used.")
 
-    # Default to master to support older, will eventually change to main
-    pull_request_branch = os.environ.get("PULL_REQUEST_BRANCH", "master")
+    # Default to project default branch if none provided
+    pull_request_branch = os.environ.get("PULL_REQUEST_BRANCH", find_default_branch())
     print("Pull requests will go to %s" % pull_request_branch)
 
     # Pull request draft