xdiff: print post-image for common records instead of pre-image

xdiff: print post-image for common records instead of pre-image

Normally it doesn't matter if we show the pre-image or th post-image
for the common parts of a diff because they are the same.  If
white-space changes are ignored they can differ, though.  The
new text after applying the diff is more interesting in that case,
so show that instead of the old contents.

Note: GNU diff shows the pre-image.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>