Adds new prompt methods on Git lib

Modifies the Git lib file (lib/git.zsh), adding three new prompt
methods:

- git_prompt_ahead(): Shows the content of the custom var
  $ZSH_THEME_GIT_PROMPT_AHEAD if the local repository has
  commits ahead from the remote origin repository

- git_prompt_short_sha(): Shows last commit SHA hash in short
  mode wrapped between the content of the custom vars
  $ZSH_THEME_GIT_PROMPT_SHA_BEFORE and
  $ZSH_THEME_GIT_PROMPT_SHA_AFTER

- git_prompt_long_sha(): Shows last commit SHA hash in long
  mode wrapped between the content of the custom vars
  $ZSH_THEME_GIT_PROMPT_SHA_BEFORE and
  $ZSH_THEME_GIT_PROMPT_SHA_AFTER