Add diff-delta code from git.git
util: add git__compress()
git_odb_object_free: don't segfault w/ arg == NULL
Add git_tag_foreach
Enable pthread condition vars
gsoc-pack-objects WIP
pack-objects: compute_write_order
Set up framework for fast import APIs

This comes from working together with @arrbee on adding
fast import APIs into libgit2.  This is just an initial
framework onto which we can add the full range of commands.
Merge remote-tracking branch 'schu/gsoc-pack-objects-threaded' into fast-import