fix symbolic ref.

this bug has been introduced by 691f6e8b39b26ff4176486e1db1d786cfe8ca6de
fix .git/index.lock conflict

__git_prompt uses "git status" which is using the lock file for a
very short momement. If there is a git command (e.g. git rebase)
running in a repository, just a simultaneously cd into that repository
from another shell can kill the rebase.
GREP_OPTIONS: -E removed as it may break existing scripts.

(eg. "[0-9]\+", which is "[0-9]+" with -E)
Merge remote-tracking branch 'DrVanScott/master'

* DrVanScott/master:
  GREP_OPTIONS: -E removed as it may break existing scripts.
  fix .git/index.lock conflict
  fix symbolic ref.
[+]: sync ".osx" with "mathiasbynens/dotfiles"

TODO: need some testing with OSX :)