Workaround for git status on old git

This sort of thing has been discussed before:
https://github.com/robbyrussell/oh-my-zsh/commit/3c87d483628267e48fc0f462f46488dcd4f87810
https://github.com/robbyrussell/oh-my-zsh/issues/40

Basically, some systems I'm working on have a version of git too old to support the `--short` format, so I added in the old status comparison removed in [this commit](https://github.com/robbyrussell/oh-my-zsh/commit/3c87d483628267e48fc0f462f46488dcd4f87810
).  The systems I'm working on are running git `1.6.0.4` and `1.6.3.3`, so I gated the old-style status checking to `<1.6.4`.  I couldn't find documentation anywhere of when the `--short` format was first supported, so that version comparison can and should be changed if someone knows better.  Also, I'm not a shell scripting maven, so it's possible my code is horribly unconventional or wrong, and I'd appreciate feedback on that.  Thanks.