mingw (git_terminal_prompt): turn on echo explictly

It turns out that when running in a Powershell window, we need to turn
on ENABLE_ECHO_INPUT because the default would be *not* to echo
anything.

This also ensures that we use the input mode where all input is read
until the user hits the Return key.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho