Update libgit2 source code (5915d70)

* Update libgit2 source code to commit
  5915d7001b0084356ece3f23c5e7c3f2be17e39c

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 patch to build winhttp.c with mingw-w64

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

* src/git2r.c (R_init_git2r): git_threads_init renamed to
  git_libgit2_init.

* src/git2r_merge.c: git_merge_head renamed to git_annotated_commit.

* src/git2r_branch.c, git2r_push.c and git2r_remote.c: git_remote_load
  renamed to git_remote_lookup.

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