git add -p: new "quit" command at the prompt.

git add -p: new "quit" command at the prompt.

There's already 'd' to stop staging hunks in a file, but no explicit
command to stop the interactive staging (for the current files and the
remaining ones).  Of course you can do 'd' and then ^C, but it would be
more intuitive to allow 'quit' action.

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