git/config: 256 color mode for diffs

TIL that this has always worked, just never documented until 2.3.
(In Git 2.3, "#rrggbb" will be supported as well.)

Unfortunately, currently Git merges everything into one CSI seq, while
256-color and true-color codes must be separate. Currently this is okay
as long as the colors aren't mixed with any attributes (bold, etc.) but
should be fixed.