git-difftool: allow skipping file by typing 'n' at prompt

git-difftool: allow skipping file by typing 'n' at prompt

This is useful if you forgot to restrict the diff to the paths you want
to see, or selecting precisely the ones you want is too much typing.

[jc: with a change to return from the function upon 'n' by Charles Bailey
and a small tweak in stdin_doesnot_contain() in the test]

Signed-off-by: Sitaram Chamarty <sitaram@atc.tcs.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>