graph: add a 2 commit buffer for lookahead

graph: add a 2 commit buffer for lookahead

In a subsequent commit the graph renderer needs to know if the next
commit is a visual root or if it is the last commit to be shown. This
requires peeking 2 commits ahead.

Commits are pre-fetched in get_revision() through next_commit_to_show()
where they are also marked as SHOWN, regardless the source they come
from.

Update graph_is_interesting() so it considers commits inside the
lookahead buffer as interesting as well.

Helped-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Pablo Sabater <pabloosabaterr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>