Improve .git-wrapper for the new GIT_TERMINAL_PROMPT var

[leehambley]

See https://github.com/blog/1957-git-2-3-has-been-released

The credential subsystem is now friendlier to scripting

When Git needs a password (e.g., to connect to a remote repository over http), it uses the credential subsystem to query any helpers (like the OS X Keychain helper), and then finally prompts the user on the terminal. When Git is run from an automated process like a cron job, there is usually no terminal available and Git will skip the prompt. However, if there is a terminal available, Git may hang forever, waiting for the user to type something. Scripts which do not expect user input can now set GIT_TERMINAL_PROMPT=0 in the environment to avoid this behavior.