Retrieve the name of the remote's default branch
The default branch of a repository is the branch which HEAD points to. If the remote does not support reporting this information directly, it performs the guess as git does; that is, if there are multiple branches which point to the same commit, the first one is chosen. If the master branch is a candidate, it wins.
This function must only be called after connecting.
0, GIT_ENOTFOUND if the remote does not have any references or none of them point to HEAD's commit, or an error message.