rev-list: skip bitmap traversal for --left-right

Running: git rev-list --left-right --use-bitmap-index one...two will produce output without any left-right markers, since the bitmap traversal returns only a single set of reachable commits. Instead we should refuse to use bitmaps here and produce the correct output using a traditional traversal. This is probably not the only remaining option that misbehaves with bitmaps, but it's particularly egregious in that it feels like it _could_ work. Doing two separate traversals for the left/right sides and then taking the symmetric set differences should yield the correct answer, but our traversal code doesn't know how to do that. It's not clear if naively doing two separate traversals would always be a performance win. A traditional traversal only needs to walk down to the merge base, but bitmaps always fill out the full reachability set. So depending on your bitmap coverage, we could end up walking old bits of history twice to fill out the same uninteresting bits on both sides. We'd also of course end up with a very large --boundary set, if the user asked for that. So this might or might not be something worth implementing later. But for now, let's make sure we don't produce the wrong answer if somebody tries it. The test covers this, but also the same thing with "--count" (which is what I originally tried in a real-world case). Ironically the try_bitmap_count() code already realizes that "--left-right" won't work there. But that just causes us to fall back to the regular bitmap traversal code, which itself doesn't handle counting (we produce a list of objects rather than a count). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Taylor Blau <me@ttaylorr.com>