diff.c: color moved lines differently, plain mode

diff.c: color moved lines differently, plain mode

Add the 'plain' mode for move detection of code. This omits the checking
for adjacent blocks, so it is not as useful. If you have a lot of the
same blocks moved in the same patch, the 'Zebra' would end up slow as it
is O(n^2) (n is number of same blocks). So this may be useful there and
is generally easy to add. Instead be very literal at the move detection,
do not skip over short blocks here.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>