Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

The posix-spawn gem's `chdir:` option is not thread-safe.

Grit guesses the `working_dir` for each repo it opens; if it can't find
one Repo#working_dir is nil. This change passes Repo#working_dir to Git
command in Git#native via Git's `--work-tree` command-line option.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

The following tests failed with git 1.7.2.1

* grit/test/test_commit.rb:201
* grit/test/test_repo.rb:422

The minimum version passing tests is 1.7.4.6