Update libgit2 source code (9e44289)

* Update libgit2 source code to commit
  9e44289c8dfece4171c7f272389b14d040c72228

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Handle changes in libgit2 API

* src/git2r_remote.c (git2r_remote_remove): The new API takes the name
  of the remote to delete

* src/git2r_reset.c (git2r_reset): Add new checkout options argument
  as NULL.

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