diff: add an internal option to dual-color diffs of diffs

When diffing diffs, it can be quite daunting to figure out what the heck
is going on, as there are nested +/- signs.

Let's make this easier by adding a flag in diff_options that allows
color-coding the outer diff sign with inverted colors, so that the
preimage and postimage is colored like the diff it is.

Of course, this really only makes sense when the preimage and postimage
*are* diffs. So let's not expose this flag via a command-line option for
now.

This is a feature that was invented by git-tbdiff, and it will be used
by `git range-diff` in the next commit, by offering it via a new option:
`--dual-color`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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