Add reflog arguments msg and who to git2r_push

* src/git2r_push.c/h (git2r_push): Add argument msg and who to
  populate reflog when updating remote tips after a push.

* R/push.r (push): Add argument msg ("update by push") and
  who (default_signature) when calling git2r_push.

* src/git2r.c: Update callMethods
Add test cases to check reflog after push