Read and write submodule branch Option #1301

[TerRoshak]

Allows for reading and writing the branch option from .gitmodules requested a while ago in #1301

submodule.Branch can be read (string) and set (string) to change the branch option in .gitmodules If a branch option was not set Branch will return null (libgit2 submodule_branch return value)

This helps in automated environments to checkout the correct tip (e.g. specific feature branch) and automatically publish changes back to the correct branch.