Avoids starting a /bin/sh and /usr/bin/env process on each
native command invocation, and will allow exec'ing the
command directly.

This removes some overhead from all native git calls in the
following ways:

 - Removes a fork previously performed by Open3, which double
   forks to avoid needing to Process::wait.
 - Removes the need to shell escape arguments, since the git
   process's argv is passed explicitly as an array.
 - Removes the /bin/sh process (1 fork/exec)

Additionally, these changes allow obtaining the git process's exit
status, available as $? after any native git command invocations.

This is mostly so it works over RPC.

Pretty awesome. And the select(2) based implementation will fix a
long-standing bug where the grit process will hang when a git
process writes more than PIPE_BUF bytes to stderr or when the input
written to the git process's stdin exceeds PIPE_BUF. The old popen3
based logic writes all of stdin, then reads all of stdout, then
reads all of stderr so everything except stdout had to come in under
PIPE_BUF. This hasn't been much of an issue but is critical to our
plans on using `git cat-file --batch' and writing a bunch of SHA1s
on stdin.

Also moving toward using a common spawn method interface that's a
compatible subset of the Process.spawn method built into Ruby >=
1.9.1. The hope is that most non-MRI platforms will eventually
support Process.spawn out of the box and the ones that don't have
backports.