git_handler.import_commits: pass in refs explicitly

This matches the way the other callers call import_git_objects.
git_handler.import_git_objects: require all arguments

All this function's callers now pass in all arguments.
git_handler.get_git_incoming: stop setting the default for refs

Because of the previous patches, refs can never be None at this point.
git2hg.extract_hg_metadata: use None as default branch value

Using False for this is just weird.
git_handler.import_git_commit: drop double setting of extra['branch']

This is set again just a few lines above.