Add C function git2r_repository_fetch_heads

* src/git2r_repository.c/h (git2r_repository_fetch_heads): Add
  function to get entries in FETCH_HEAD file.

* src/git2r.c: Add git2r_repository_fetch_heads to callMethods.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add method fetch_heads

* R/fetch.r (fetch_heads): Get updated heads during the last fetch.

* man/fetch_heads-methods.Rd: Add documentation

* NAMESPACE: Update

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Update NEWS

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>