How to use the pbr.git function in pbr

def _from_git(distribution):
    option_dict = distribution.get_option_dict('pbr')
    changelog = git._iter_log_oneline()
    if changelog:
        changelog = git._iter_changelog(changelog)
    git.write_git_changelog(option_dict=option_dict, changelog=changelog)
    git.generate_authors(option_dict=option_dict)

def _from_git(distribution):
    option_dict = distribution.get_option_dict('pbr')
    changelog = git._iter_log_oneline()
    if changelog:
        changelog = git._iter_changelog(changelog)
    git.write_git_changelog(option_dict=option_dict, changelog=changelog)
    git.generate_authors(option_dict=option_dict)

def _get_version_from_git(pre_version=None):
    """Calculate a version string from git.

    If the revision is tagged, return that. Otherwise calculate a semantic
    version description of the tree.

    The number of revisions since the last tag is included in the dev counter
    in the version for untagged versions.

    :param pre_version: If supplied use this as the target version rather than
        inferring one from the last tag + commit messages.
    """
    git_dir = git._run_git_functions()
    if git_dir:
        try:
            tagged = git._run_git_command(
                ['describe', '--exact-match'], git_dir,
                throw_on_error=True).replace('-', '.')
            target_version = version.SemanticVersion.from_pip_string(tagged)
        except Exception:
            if pre_version:
                # not released yet - use pre_version as the target
                target_version = version.SemanticVersion.from_pip_string(
                    pre_version)
            else:
                # not released yet - just calculate from git history
                target_version = None
        result = _get_version_from_git_target(git_dir, target_version)
        return result.release_string()
    # If we don't know the version, return an empty string so at least
    # the downstream users of the value always have the same type of
    # object to work with.

def _get_revno(git_dir):
    """Return the commit count since the most recent tag.

    We use git-describe to find this out, but if there are no
    tags then we fall back to counting commits since the beginning
    of time.
    """
    raw_tag_info = git._get_raw_tag_info(git_dir)
    if raw_tag_info:
        return raw_tag_info

    # no tags found
    revlist = git._run_git_command(
        ['rev-list', '--abbrev-commit', 'HEAD'], git_dir)
    return len(revlist.splitlines())