branch upstream_name InvalidRefSpec error on literal remote values for branch

when using hub (https://hub.github.com) to checkout pull requests, it generates a config entry for the branch with the direct reference to the remote git url instead of adding in a remote. This seems to cause some confusion for pygit, specifically iy will end up throwing an InvalidSpecError as pygit tries to follow the remote as a name instead of being to handle a literal git repo value. I'd suggest in such a case pygit simply return the remote name.

https://github.com/libgit2/pygit2/blob/master/src/branch.c#L245

$ branch
<_pygit2.Branch object at 0x10d1197b0>
$ branch.upstream_name
*** _pygit2.InvalidSpecError: 'https://github.com/steveparker-1984/cloud-custodian.git' is not a valid remote name.

entry in .git/config

[branch "fail-on-assume-role-error"]
        remote = https://github.com/steveparker-1984/cloud-custodian.git
        merge = refs/heads/fail-on-assume-role-error

Comienza inspeccionando src/branch.c alrededor de la línea 245 y reproduce la configuración mostrada con un valor HTTPS literal en branch.remote. Rastrea el manejo de branch.upstream_name para ese valor; se considera terminado cuando ya no genera InvalidSpecError y sigue el comportamiento solicitado para el nombre del remoto.