We were spending a lot of time forking/execing git-cat-file and
git-hash-object. We now maintain a global Git repository object in order to use
Git.pm's more efficient hash_and_insert_object and cat_blob methods.
Signed-off-by: Adam Roben <aroben@apple.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>