Display the diffs for a merge in a unified fashion.

Stuff that ended up in the result is shown in bold with a "+" at the
beginning of the line; stuff that didn't is in the normal font with
a "-" at the beginning of the line.  The color shows which parent
the stuff was in; red for the first parent, blue for the second, then
green, purple, brown, and the rest are grey.  If the result is different
from all of the parents it is shown in black (and bold).