Add one more branch test case
More argument checks in git2r_branch_list C function
Refactoring of git2r_branch_list C function

* src/git2r_branch.c (git2r_branch_init): Init slots in S4 class
  git_branch

* src/git2r_branch_list.c (git2r_branch_list): Split function and
  initialize S4 class git_branch with git2r_branch_init.