This CL keeps disallowing `go get` from falling to the prompt unless
user has set GIT_TERMINAL_PROMPT env variable. If GIT_TERMINAL_PROMPT
is set, go-get will not override its value and will prompt for
username/password in the case of GIT_TERMINAL_PROMPT=1.

Fixes #12706.

Change-Id: Ibd6b1100af6b04fb8114279cdcf608943e7765be
Reviewed-on: https://go-review.googlesource.com/16091Reviewed-by: Andrew Gerrand <adg@golang.org>