Support alternate target branch

The `repo.branch` configuration setting controls the branch of the
repote repository to check out.  It is also used as the target
branch name for the Gitea pull request.

updatebot.py

@@ -158,6 +158,7 @@ Project = Union[
 class RepoConfig(pydantic.BaseModel):
     url: str
     token_file: Path
+    branch: str = 'master'
 
     @functools.cached_property
     def repo_api_url(self) -> str:
@@ -300,7 +301,7 @@ def main() -> None:
         d = Path(d)
         log.debug('Retreiving repository Git URL')
         repo_url = config.repo.get_git_url()
-        repo = git.Repo.clone_from(repo_url, d, depth=1)
+        repo = git.Repo.clone_from(repo_url, d, depth=1, b=config.repo.branch)
         log.debug('Checking out new branch: %s', args.branch_name)
         repo.heads[0].checkout(force=True, B=args.branch_name)
         title = None
@@ -321,7 +322,7 @@ def main() -> None:
             )
         )
         repo.remote().push(force=True)
-        config.repo.create_pr(title, args.branch_name, 'master')
+        config.repo.create_pr(title, args.branch_name, config.repo.branch)
 
 
 if __name__ == '__main__':