Misc patches for git completion and vcs_info

[Marc Finet <m.dreadlock@xxxxxxxxx>]

Hello,

Since i switched to zsh, i've being trying to tweak my dev environment and
fell on a few zsh behaviors i did not expect especially regarding git
handling (i played a lot with interesting/unique features of zsh though).

Below is a list of patches that i added in my current zsh (based on
tonight's master). Please consider them for review as my zsh-level is
currently very low (e.g. patch 2) and i might have broken other use-cases
(patch 4 for instance makes me a little bit nervous as it seems to easy).

Thanks,

Marc.