This is a transitionary interface which does NOT require an
event loop. It can be plugged into in current synchronous code
without major surgery.
It allows HTTP/1.1 pipelining-like functionality by taking
advantage of predictable and well-specified POSIX pipe semantics
by stuffing multiple git cat-file requests into the --batch pipe
With xt/git_async_cmp.t and GIANT_GIT_DIR=git.git, the async
interface is 10-25% faster than the synchronous interface since
it can keep the "git cat-file" process busier.
This is expected to improve performance on systems with slower
storage (but multiple cores).