Set the work_tree to the working_dir

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.
Remove use of posix-spawn chdir:

The posix-spawn gem's `chdir:` option is not thread-safe.
Update History.txt for 2.7.0
Merge branch 'work_tree' into 'master'

Use git --work-tree when there is a working directory

See merge request !5