zshrc: changing invocation of vcs_info()

zshrc: changing invocation of vcs_info()

Calling vcs_info() from PS1 is a stupid idea, because it will be called
*every* *single* *time* the prompt is redrawn.

That's among other things, every time you hit tab. Annoying.

This calls vcs_info() from precmd(). That's a *lot* less than before.