Update libgit2 source code (0bb237e)

* Update libgit2 source code to commit
  0bb237ed89a6700a1b6b884444ff044fd1467535

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

* Update patches that are applied to libgit2 to build as an R package.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Update Makevars.in and Makevars.win

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Update to handle changes in libgit2 push api

* src/git2r_push.c (git2r_push): Remove call to removed libgit2
  git_push_unpack_ok.

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