add -p: 'q' should really quit

add -p: 'q' should really quit

The "quit" command was added in 9a7a1e0 (git add -p: new "quit" command at
the prompt, 2009-04-10) to allow the user to say that hunks other than
what have already been chosen are undesirable, and exit the interactive
loop immediately.  It forgot that there may be an undecided hunk before
the current one.  In such a case, the interactive loop still goes back to
the beginning.

Clear all the USE bit for undecided hunks and exit the loop.

Signed-off-by: Junio C Hamano <gitster@pobox.com>