How to use the ghstack.diff.Diff function in ghstack

def branch_head(username: str, ghnum: GhNumber) -> GitCommitHash:
    return branch(username, ghnum, "head")


def branch_orig(username: str, ghnum: GhNumber) -> GitCommitHash:
    return branch(username, ghnum, "orig")


STACK_HEADER = "Stack from [ghstack](https://github.com/ezyang/ghstack)"


@dataclass
class DiffWithGitHubMetadata:
    diff: ghstack.diff.Diff
    number: GitHubNumber
    # Really ought not to be optional, but for BC reasons it might be
    remote_source_id: Optional[str]
    title: str
    body: str
    closed: bool
    ghnum: GhNumber
    pull_request_resolved: ghstack.diff.PullRequestResolved


def main(msg: Optional[str],
         username: str,
         github: ghstack.github.GitHubEndpoint,
         update_fields: bool = False,
         sh: Optional[ghstack.shell.Shell] = None,
         stack_header: str = STACK_HEADER,

# The orig commit of the prev diff we submitted.  This
    # corresponds to the 'orig' branch in GH.
    base_orig: GitCommitHash

    # Message describing the update to the stack that was done
    msg: Optional[str]

    # Description of all the diffs we submitted; to be populated
    # by Submitter.
    stack_meta: List[DiffMeta]

    # List of input diffs which we ignored (i.e., treated as if they
    # did not exist on the stack at all), because they were associated
    # with a patch that contains no changes.  GhNumber may be false
    # if the diff was never associated with a PR.
    ignored_diffs: List[Tuple[ghstack.diff.Diff, Optional[GitHubNumber]]]

    # List of diffs to process, in chronological order
    stack: List[ghstack.diff.Diff]

    # Set of seen ghnums
    seen_ghnums: Set[GhNumber]

    # String used to describe the stack in question
    stack_header: str

    # Clobber existing PR description with local commit message
    update_fields: bool

    # Print only PR URL to stdout
    short: bool

def convert(h: CommitHeader) -> ghstack.diff.Diff:
        parents = h.parents()
        if len(parents) != 1:
            raise RuntimeError(
                "The commit {} has {} parents, which makes my head explode.  "
                "`git rebase -i` your diffs into a stack, then try again."
                .format(h.commit_id(), len(parents)))
        return ghstack.diff.Diff(
            title=h.title(),
            summary=h.commit_msg(),
            oid=h.commit_id(),
            source_id=h.tree(),
            pull_request_resolved=ghstack.diff.PullRequestResolved.search(h.raw_header),
            patch=GitPatch(h)
        )