Accept no-mark style for normal diff input.

This makes it easier to compare two lines just adding 1c1 and -- line.
Actually, this worked before accidentally, but insert > and < mark
at the top of lines.  Now the data itself remains unchanged.
add conflict marker handling
support git-diff --combined format
handle diff3 original text
more precise 7 level control of combined diff format
change &compare interface, and compare priority
use compare() interface everywhere.
document udpate