Change to use omz git_prompt* functions

Removed references for official git __git_ps1() functions.
Change to use omz function git_prompt* functions like:
- git_prompt_info() -> branch name
- git_prompt_status() -> check if untracked, modified, added,
  stashed file are in the current git repo
- git_remote_status() -> check if current repo is ahead, behind,
  or diverged.
These functions returns string automatically added to your PS1;
these are customizable via exporting ZSH_THEME_GIT_PROMPT*