range-diff: use dim/bold cues to improve dual color mode

It *is* a confusing thing to look at a diff of diffs. All too easy is it
to mix up whether the -/+ markers refer to the "inner" or the "outer"
diff, i.e. whether a `+` indicates that a line was added by either the
old or the new diff (or both), or whether the new diff does something
different than the old diff.

To make things easier to process for normal developers, we introduced
the dual color mode which colors the lines according to the commit diff,
i.e. lines that are added by a commit (whether old, new, or both) are
colored in green. In non-dual color mode, the lines would be colored
according to the outer diff: if the old commit added a line, it would be
colored red (because that line addition is only present in the first
commit range that was specified on the command-line, i.e. the "old"
commit, but not in the second commit range, i.e. the "new" commit).

However, this dual color mode is still not making things clear enough,
as we are looking at two levels of diffs, and we still only pick a color
according to *one* of them (the outer diff marker is colored
differently, of course, but in particular with deep indentation, it is
easy to lose track of that outer diff marker's background color).

Therefore, let's add another dimension to the mix. Still use
green/red/normal according to the commit diffs, but now also dim the
lines that were only in the old commit, and use bold face for the lines
that are only in the new commit.

That way, it is much easier not to lose track of, say, when we are
looking at a line that was added in the previous iteration of a patch
series but the new iteration adds a slightly different version: the
obsolete change will be dimmed, the current version of the patch will be
bold.

At least this developer has a much easier time reading the range-diffs
that way.

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

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