Fix reflog message for fast-forward merge after pull

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add internal function to merge fetch head into HEAD

* src/git2r_merge.c/h (git2r_merge_fetch_heads): Merge the given fetch
  head data into HEAD.

* src/git2r.c: Add 'git2r_merge_fetch_heads' to callMethods.

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