commit-reach: stale commits may prune generation further

commit-reach: stale commits may prune generation further

The remove_redundant_with_gen() algorithm performs a depth-first-search
to find commits in the 'array' list, starting at the parents of each
commit in 'array'. The result is that commits in 'array' are marked
STALE when they are reachable from another commit in 'array'.

This depth-first-search is fast when commits lie on or near the
first-parent history of the higher commits. The search terminates early
if all but one commit becomes marked STALE.

However, it is possible that there are two independent commits with high
generation number. In that case, the depth-first-search might languish
by searching in lower generations due to the fixed min_generation used
throughout the method.

With the expectation that commits with lower generation are expected to
become STALE more often, we can optimize further by increasing that
min_generation boundary upon discovery of the commit with minimum
generation.

We must first sort the commits in 'array' by generation. We cannot sort
'array' itself since it must preserve relative order among the returned
results (see revision.c:mark_redundant_parents() for an example).

This simplifies the initialization of min_generation, but it also allows
us to increase the new min_generation when we find the commit with
smallest generation remaining.

This requires more than two commits in order to test, so I used the
Linux kernel repository with a few commits that are slightly off of the
first-parent history. I timed the following command:

  git merge-base --independent 2ecedd756908 d2360a398f0b \
	1253935ad801 160bab43419e 0e2209629fec 1d0e16ac1a9e

The first two commits have similar generation and are near the v5.10
tag. Commit 160bab43419e is off of the first-parent history behind v5.5,
while the others are scattered somewhere reachable from v5.9. This is
designed to demonstrate the optimization, as that commit within v5.5
would normally cause a lot of extra commit walking.

Since remove_redundant_with_alg() is called only when at least one of
the input commits has a finite generation number, this algorithm is
tested with a commit-graph generated starting at a number of different
tags, the earliest being v5.5.

commit-graph at v5.5:

 | Method                | Time  |
 |-----------------------+-------|
 | *_no_gen()            | 864ms |
 | *_with_gen() (before) | 858ms |
 | *_with_gen() (after)  | 810ms |

commit-graph at v5.7:

 | Method                | Time  |
 |-----------------------+-------|
 | *_no_gen()            | 625ms |
 | *_with_gen() (before) | 572ms |
 | *_with_gen() (after)  | 517ms |

commit-graph at v5.9:

 | Method                | Time  |
 |-----------------------+-------|
 | *_no_gen()            | 268ms |
 | *_with_gen() (before) | 224ms |
 | *_with_gen() (after)  | 202ms |

commit-graph at v5.10:

 | Method                | Time  |
 |-----------------------+-------|
 | *_no_gen()            |  72ms |
 | *_with_gen() (before) |  37ms |
 | *_with_gen() (after)  |   9ms |

Note that these are only modest improvements for the case where the two
independent commits are not in the commit-graph (not until v5.10). All
algorithms get faster as more commits are indexed, which is not a
surprise. However, the cost of walking extra commits is more and more
prevalent in relative terms as more commits are indexed. Finally, the
last case allows us to jump to the minimum generation between the last
two commits (that are actually independent) so we greatly reduce the
cost in that case.

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