Moving commits runs a rebase, which can take a while. Instead of
letting the drop indicator vanish the moment the button is released,
keep it in place and turn it into a "moving commits here" spinner once
the move takes longer than a short grace period, so that quick moves
stay free of flicker. The indicator is cleared when the post-move
refresh lands.
Render the insertion point of a commit drag as a non-model item in the
commits list. It must be inserted at the right position relative to
the section headers, because the list renderer assumes non-model items
are ordered by their model index.
Not used yet, we'll hook it up to the drag gesture in the next commit.
