Get configured upstream canonical name from local branch

* src/git2r_branch.c/h (git2r_branch_upstream_canonical_branch): Get
  the configured canonical name of the upstream branch, given a local
  branch, i.e "branch.branch_name.merge" property of the config file.

* src/git2r.c: Add git2r_branch_upstream_canonical_branch to callMethods
Get canonical name from branch

* src/git2r_branch.c/h (git2r_branch_canonical_name): Get the full
  name of a branch.

* src/git2r.c: Add git2r_branch_canonical_name to callMethods
Fix push of S4 class git_branch
Add push branch example