Add ZSH autocomplete compatibility.

Add a couple of lines that ensure that tab completions are run properly in zsh. Otherwise, pressing tab will cause a mess of newlines, error messages, and missing prompts.

Below is an example of bad autocomplete behavior. `tig <TAB>` was typed, and while the desired result is there (the file listing), the additional error messages make it very hard to work with tig, especially on larger projects and longer commands.

![Image of bad tab completion](https://f.cloud.github.com/assets/550023/2058828/c05875d4-8b86-11e3-817b-3f188b9f873e.png)