Update libgit2 source code (fc6ac07)

* Update libgit2 source code to commit
  fc6ac074ee7ea945819e0a1d6f65ba160ba403d7

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Fix R CMD check warning

* src/libgit2/checkout.c (checkout_conflict_append_remove): After
  update of libgit2 source code, change 'abort' to 'Rf_error' to fix R
  CMD check compiled code warning.

* patches/checkout-pass-R-CMD-check-git2r.patch: Add patch

* Makefile: Update target 'sync_libgit2' to apply patch.

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