Added target in Makefile to sync libgit2 source code

* Makefile: Added target to copy and sync files from a local libgit2
  repository
Updated libgit2 source code (bcc6229)

* Updated libgit2 source code to commit
  bcc622934a90307390f058c7ef4203f8bbcfe0d6
Updated Makevars.in/win to build updated libgit2
Fixed git2r to build and pass tests with updated libgit2