commit: use generation number in remove_redundant()

The static remove_redundant() method is used to filter a list
of commits by removing those that are reachable from another
commit in the list. This is used to remove all possible merge-
bases except a maximal, mutually independent set.

To determine these commits are independent, we use a number of
paint_down_to_common() walks and use the PARENT1, PARENT2 flags
to determine reachability. Since we only care about reachability
and not the full set of merge-bases between 'one' and 'twos', we
can use the 'min_generation' parameter to short-circuit the walk.

When no commit-graph exists, there is no change in behavior.

For a copy of the Linux repository, we measured the following
performance improvements:

git merge-base v3.3 v4.5

Before: 234 ms
 After: 208 ms
 Rel %: -11%

git merge-base v4.3 v4.5

Before: 102 ms
 After:  83 ms
 Rel %: -19%

The experiments above were chosen to demonstrate that we are
improving the filtering of the merge-base set. In the first
example, more time is spent walking the history to find the
set of merge bases before the remove_redundant() call. The
starting commits are closer together in the second example,
therefore more time is spent in remove_redundant(). The relative
change in performance differs as expected.

Reported-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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