head_repo missing from create_pull

[xmo-odoo]

A while ago, Github allowed forking repositories within an organisation (with both source and destination being the organisation).

When creating a PR, the head can be {owner}:{branch} which is not sufficient to distinguish between same-organisation forks. To handle this issue, they added a head_repo parameter, which is the name of the source repository for the PR.

pygithub does not currently support this parameter. It should probably be either a string or an organisation.

An other update which might be nice is to allow using a Branch as head, that way pygithub would have all the metadata for both sides of the PR creation and can infer all the bits and where to put them.