Execute native git commands directly

[rtomayko]

This removes as many as 3 fork/exec's from all native git calls, makes it possible to retrieve the git process's exit status, removes the need to shell escape command arguments, and allows setting environment variables on the child process without changing the parent process's environment.

Previously, #native calls created a process hierarchy like this:

grit ruby process
- <fork again to reparent under init>
  - /bin/sh -c '/usr/bin/env git ...'
    - /usr/bin/env ...
      - /usr/local/bin/git ...

This #native implementation forks once and execs git directly:

grit ruby process
  - /usr/local/bin/git ...

Some light benchmarks shows decent gains.

Linux:

$ uname -a
Linux fe1.rs.github.com 2.6.26-2-amd64 1 SMP Thu Sep 16 15:56:38 UTC 2010 x86_64 GNU/Linux
$ ruby -v
ruby 1.8.7 (2008-08-11 patchlevel 72) [x86_64-linux]
$ ruby native-benchmark.rb 
                                    user     system      total        real
sh                              0.510000   2.170000   5.520000 ( 13.555887)
execute                         0.240000   1.280000   5.620000 (  6.161315)

Mac:

$ uname -a
Darwin asha.local 10.4.0 Darwin Kernel Version 10.4.0: Fri Apr 23 18:28:53 PDT 2010; root:xnu-1504.7.4~1/RELEASE_I386 i386
$ ruby -v
ruby 1.8.7 (2010-06-23 patchlevel 299) [i686-darwin10.4.0]
$ ruby native-benchmark.rb 
                                    user     system      total        real
sh                              0.440000   2.510000   3.520000 (  8.327763)
execute                         0.330000   2.370000   4.860000 (  5.868368)

Even without the performance benefits, the ability to retrieve exit status and setup the child's environment are much needed additions that will let us clean up some janky code elsewhere.

The old Open3 based Git#sh and Git#run methods remain and are used as a fallback for non-POSIX systems (I assume Open3 works on platforms without fork(2) somehow) and also in cases where a #native is called with a pipeline as the last argument.

I'll be testing this out in GitHub staging/production environments this week.

[rtomayko]

This is ready:

• Reduces number of fork/execs by between 2-4. Nice perf increase.
• Removes the need to shell quote input arguments.
• Ability to retrieve child process exit status.
• Ability to set child process environment without effecting parent process.
• Ability to set child process working dir / pwd without effecting parent process.
• Handles arbitrarily large input, output, and error streams. We can pipe stuff into commands without temp files and without worrying about hitting PIPE_BUF.
• Based around Ruby 1.9's Process::spawn which will hopefully be a portability win in the future when support is added to JRuby/Windows.

Tested under various versions of MRI 1.8.7, REE 1.8.7, and 1.9.2-p0. Not all of these features work under JRuby yet but it works as well as it did previously.