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)