(Bash) Do not use `readlink -f` for Mac OSX
Add 'git/git' into submodules
(Git) Use git-completion.bash and git-prompt.sh from git repository directly
(Bash) Add git-prompt-custom.sh contain git_branch(),
git_since_last_commit()

1. git_branch() to get current branch.
2. git_since_last_commit() to get how long before the last commit .
(Bash) update shell-prompt.sh, remove powerline symbols