In response to issue #60 (git_index_write really slow), the write_index
function has been rewritten to improve its performance -- it should now
be in par with the performance of git.git.

On top of that, if Posix Threads are available when compiling libgit2, a
new threaded writing system will be used (3 separate threads take care
of solving byte-endianness, hashing the contents of the index and
writing to disk, respectively). For very long Index files, this method
is up to 3x times faster than git.git.

Signed-off-by: Vicent Marti <tanoku@gmail.com>