Better support Bash 4.4 by moving default prompt to GIT_PROMPT

[juniorbird]

I was vexed that the un-parsed prompt was always showing before any command was executed in my home laptop, while everything worked perfectly on all my work machines.

Then I found that, as of Bash 4.4, there's a real PS0 -- and it runs before the terminal command is executed.

Rewrote this script to prefer GIT_PROMPT but fall back to PS0 for people not on Bash 4.4. It'll still be annoying for anyone who upgrades their Bash, but there's not much to be done -- I spent a while trying to reliably tell what shell the user was in, but found I could reliably trick perl by having subshells run Bash 3.2.

PR updates documentation as well.