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>