commit-reach: use heuristic in remove_redundant()

commit-reach: use heuristic in remove_redundant()

Reachability algorithms in commit-reach.c frequently benefit from using
the first-parent history as a heuristic for satisfying reachability
queries. The most obvious example was implemented in 4fbcca4e
(commit-reach: make can_all_from_reach... linear, 2018-07-20).

Update the walk in remove_redundant() to use this same heuristic. Here,
we are walking starting at the parents of the input commits. Sort those
parents and walk from the highest generation to lower. Each time, use
the heuristic of searching the first parent history before continuing to
expand the walk.

The order in which we explore the commits matters, so update
compare_commits_by_gen to break generation number ties with commit date.
This has no effect when the commits are in a commit-graph file with
corrected commit dates computed, but it will assist when the commits are
in the region "above" the commit-graph with "infinite" generation
number. Note that we cannot shift to use
compare_commits_by_gen_then_commit_date as the method prototype is
different. We use compare_commits_by_gen for QSORT() as opposed to as a
priority function.

The important piece is to ensure we short-circuit the walk when we find
that there is a single non-redundant commit. This happens frequently
when looking for merge-bases or comparing several tags with 'git
merge-base --independent'. Use a new count 'count_still_independent' and
if that hits 1 we can stop walking.

To update 'count_still_independent' properly, we add use of the RESULT
flag on the input commits. Then we can detect when we reach one of these
commits and decrease the count. We need to remove the RESULT flag at
that moment because we might re-visit that commit when popping the
stack.

We use the STALE flag to mark parents that have been added to the new
walk_start list, but we need to clear that flag before we start walking
so those flags don't halt our depth-first-search walk.

On my copy of the Linux kernel repository, the performance of 'git
merge-base --independent <all-tags>' goes from 1.1 seconds to 0.11
seconds.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>