Restore environments after run git command.

ruby-git sets following environments: GIT_DIR, GIT_WORK_TREE and GIT_INDEX_FILE
But it does not restore in GIT::Lib#command.
This PR fixes it.

---

before:
```sh
% ruby -rgit -e '
    p([:before_call, ENV["GIT_DIR"], ENV["GIT_WORK_TREE"]])
    g = Git.init("/tmp/foo")
    p([:after_call, ENV["GIT_DIR"], ENV["GIT_WORK_TREE"]])
  '
[:before_call, nil, nil]
[:after_call, "/tmp/foo/.git", "/tmp/foo"]
```

after:
```sh
% ruby -rgit -e '
    p([:before_call, ENV["GIT_DIR"], ENV["GIT_WORK_TREE"]])
    g = Git.init("/tmp/foo")
    p([:after_call, ENV["GIT_DIR"], ENV["GIT_WORK_TREE"]])
  '
[:before_call, nil, nil]
[:after_call, nil, nil]
```