Add force argument to git_branch_create

[M-Kusumgar]

We would like to create a branch with a particular ref and if it exists then we would like to overwrite it. libgit2::git_branch_create has a force arg that already replicates this behaviour so would just be a case of adding the force arg to the gert::git_branch_create. I am happy to do a PR on this if you don't mind me adding this into gert!

[jeroen]

Yes, you can add a PR, thanks