revision.c: discount side branches when computing TREESAME

revision.c: discount side branches when computing TREESAME

Use the BOTTOM flag to define relevance for pruning. Relevant commits
are those that are !UNINTERESTING or BOTTOM, and this allows us to
identify irrelevant side branches (UNINTERESTING && !BOTTOM).

If a merge has relevant parents, and it is TREESAME to them, then do not
let irrelevant parents cause the merge to be treated as !TREESAME.

When considering simplification, don't always include all merges -
merges with exactly one relevant parent can be simplified, if TREESAME
according to the above rule.

These two changes greatly increase simplification in limited, pruned
revision lists.

Signed-off-by: Kevin Bracey <kevin@bracey.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>