Disable git terminal prompt during fetch/clone

git fetch operation may prompt user to enter username and password.
This won't be visible to user when repo sync operation since stdout and
stderr are redirected. If that happens, user may think repo is doing
work and likely won't realize it's stuck on user's input.

This patch disables prompt for clone and fetch operations, and repo will
fail fast.

R=gavinmak@google.com

Bug: b/368644181
Change-Id: I2efa88ae66067587a00678eda155d861034b9127
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/438001


Reviewed-by: Nasser Grainawi <nasser.grainawi@linaro.org>
Tested-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>