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.

This is mostly so it works over RPC.

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.

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

Relative paths in a <repo>.git/objects/info/alternates file are to
be expanded relative to the <repo>.git/objects directory.
Previously, they were expanded relative to the <repo>.git directory.

The current code checks in both locations for backward compatibility
with grit <= 2.3.0 but I'd rather rip it out completely.