Add CONTRIBUTING.md to .Rbuildignore
Add patches to pass 'R CMD check' after updating libgit2

* To pass checking compiled code in R CMD check:

* tools/cache-pass-R-CMD-check-git2r.patch: Change all printf to Rprintf

* tools/diff_print-pass-R-CMD-check-git2r.patch: In
  git_diff_print_callback__to_file_handle return error code (-1)
  instead of using stdout if FILE pointer is NULL.

* tools/util-pass-R-CMD-check-git2r.patch: Change all printf to Rprintf

* Makefile: Apply patches after updating libgit2 with 'sync_libgit2'
  target
Update libgit2 source code (59403f1)

* Update libgit2 source code to commit
  59403f1ff55346c64bfaa0744ea7f3375da71725

* Apply patches to pass 'R CMD check git2r'