Re: [PATCH v6 2/3] revision: add peek functions for lookahead

> The solution is to skip peeking entirely and instead call
> get_revision_internal() to populate a small lookahead buffer -
> it only needs two slots.
>
>     struct git_graph {
>         // ...
>         struct commit *lookahead[2];
>         int lookahead_nr;
>     }
>
>     while (revs->graph->lookahead_nr < 2) {
>         struct commit *next = get_revision_internal(revs);
>         if (!next)
>             break;
>         graph_push_lookahead(revs->graph, next);
>     }
>
> After prototyping this locally, the three test_expect_failure
> cases in t4218 went away (though I had to do some minor tweaks
> to ensure it become fully deterministic by ticking the commit
> timestamps.
>
> One subtlety worth mentioning: get_revision_internal() sets
> SHOWN on commits, so lookahead commits are marked SHOWN before
> graph_update() processes them. This makes graph_is_interesting()
> think they are already displayed. The fix is a small check in
> graph_is_interesting() that recognizes commits in the lookahead
> buffer as interesting regardless of their SHOWN flag.
>
>     for (i = 0; i < graph->lookahead_nr; i++)
>         if (graph->lookahead[i] == commit)
>             return 1;
>     // other checks after this ...
>
> This approach ultimately removes the need for
> revision_peek_next_commit() and revision_has_commits_after()
> entirely - the graph code no longer needs to peek
> at rev_info internals.