terminal: set VMIN and VTIME in non-canonical mode

terminal: set VMIN and VTIME in non-canonical mode

If VMIN and VTIME are both set to zero then the terminal performs
non-blocking reads which means that read_key_without_echo() returns
EOF if there is no key press pending. This results in the user being
unable to select anything when running "git add -p".  Fix this by
explicitly setting VMIN and VTIME when enabling non-canonical mode.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>