Improve usability of Remote.ls_remotes's result

[jorio]

Remote.ls_remotes used to return a dict with str keys, which is inconsistent with how pygit2 generally returns structured results. This PR introduces RemoteHead, a class with fields.

Backwards compatibility with the old-style dict was kept via RemoteHead.__getitem__ which emits a DeprecationWarning.

Also, when the local flag is off, RemoteHead.loid now forwards oid zero (0000000) as returned by libgit2 instead of making it None.

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Tangent: I think ls_remotes is a bit of a confusing name. Perhaps something like list_heads or advertised_heads would be more intuitive, but this could be for another PR. What do you think?

[jdavid]

Remote.list_heads() looks good to me, since it wraps git_remote_ls which returns a list of git_remote_head