Add C function git2r_branch_set_upstream

* src/git2r_branch.c/h (git2r_branch_set_upstream): Add function to
  set the upstream configuration for a given local branch

* src/git2r.c: Add git2r_branch_set_upstream to callMethods
Add method branch_set_upstream

* R/branch.r (branch_set_upstream): Add method for signature S4 class
  git_branch

* man/branch_set_upstream-methods.Rd: Add documentation

* NAMESPACE: Update