Updated libgit2 source code (59e3f45)

* Updated libgit2 source code to commit
  59e3f45b6150984e6a74ce79062caa459eda369f

* src/libgit2 (util.c, cache.c): Changed printf to Rprintf
Fix to build with updated libgit2

* src/git2r_remotes.c (git2r_remote_rename): Fix to handle changed libgit2 API