Added libgit2 source code (4e1f517) required to build the git2r package

The source code and headers have been added to the src folder.

* src/libgit2/cache.c (git_cache_dump_stats): printf calls have been
    modified to use the R printing routine Rprintf

* src/libgit2/util.c (git__hexdump): printf calls have been modified to
    use the R printing routine Rprintf