add--interactive: respect diff.compactionHeuristic

add--interactive: respect diff.compactionHeuristic

We use plumbing to generate the diff, so it doesn't
automatically pick up UI config like compactionHeuristic.
Let's forward it on, since interactive adding is porcelain.

Note that we only need to handle the "true" case. There's no
point in passing --no-compaction-heuristic when the variable
is false, since nothing else could have turned it on.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>