git-info: modify Ctrl+C behavior to be more explicit

(Update: looks like it's related to #307)

I find git-info's auto-disable behavior confusing and too implicit. I'm referring to the case of Ctrl+C press while git-info is running.

Even though it prints a warning message once, it's easy to miss it. Happened to me, and I've also seen others with disabled-git-prompts, while they weren't aware to why it was actually disabled. The git prompt is very cool, and I believe that in many cases users don't want it to automatically go away.

I suggest to make the behavior more expected and explicit, that is:
1. Ctrl+C merely aborts the current call, doesn't modify settings, but displays an informative message.
2. Suggests running 'git-info off' to disable git prompt in current repo.
3. Suggests adding a zstyle setting to disable git prompt for all repos.

This way, the user is fully aware of the disable action.