`git describe` exit code 128 with no buffer on node 0.8.x

[cadorn]

This is evident when calling sm status --debug on package that contains git dirs.

Everything works with node 0.6.19

It starts to act up here: https://github.com/sourcemint/sm-plugin-git/blob/55cc0e86526b07f7ba34181c8cb86c9ce9e74a99/lib/git.js#L745-L761

To reproduce, run sm status --debug on sm try github.com/sourcemint/sm with some dependencies in edit mode.

[cadorn]

Problem persists with 0.8.18 & 19. Need test case or don't use exec for git calls.