add -p: disable stdin buffering when interactive.singlekey is set

add -p: disable stdin buffering when interactive.singlekey is set

The builtin "add -p" reads the key "F2" as three separate keys "^[",
"O" and "Q". The "Q" causes it to quit which is probably not what the
user was expecting. This is because it uses poll() to check for
pending input when reading escape sequences but reads the input with
getchar() which is buffered by default and so hoovers up all the
pending input leading poll() think there isn't anything pending. Fix
this by calling setbuf() to disable input buffering if
interactive.singlekey is set.

Looking at the comment above mingw_getchar() in terminal.c I wonder if
that function is papering over this bug and could be removed.
Unfortunately I don't have access to windows to test that.

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