Support for special branch names in git submodule declarations

[Byron]

Git now supports the branch field in submodule declarations, and adds a twist that is not handled by GitPython.

https://raw.githubusercontent.com/git/git/master/Documentation/RelNotes/2.10.2.txt
* Recent git allows submodule.<name>.branch to use a special token
"." instead of the branch name; the documentation has been updated
to describe it.

https://git-scm.com/docs/gitmodules
submodule.<name>.branch
A remote branch name for tracking updates in the upstream submodule. If the option is not specified, it defaults to master. A special value of . is used to indicate that the name of the branch in the submodule should be the same name as the current branch in the current repository. See the --remote documentation in git-submodule[1] for details.

When this issue is resolved, GitPython can handle special names and behaves similarly to cgit when encountering them.

Related to #545