commit-reach: fix first-parent heuristic

commit-reach: fix first-parent heuristic

The algorithm in can_all_from_reach_with_flags() performs a depth-
first-search, terminated by generation number, intending to use
a hueristic that "important" commits are found in the first-parent
history. This heuristic is valuable in scenarios like fetch
negotiation.

However, there is a problem! After the search finds a target commit,
it should pop all commits off the stack and mark them as "can reach".
This logic is incorrect, so the algorithm instead walks all reachable
commits above the generation-number cutoff.

The existing algorithm is still an improvement over the previous
algorithm, as the worst-case complexity went from quadratic to linear.
The performance measurement at the time was good, but not dramatic.
By fixing this heuristic, we reduce the number of walked commits.

We can also re-run the performance tests from commit 4fbcca4e
"commit-reach: make can_all_from_reach... linear".

Performance was measured on the Linux repository using
'test-tool reach can_all_from_reach'. The input included rows seeded by
tag values. The "small" case included X-rows as v4.[0-9]* and Y-rows as
v3.[0-9]*. This mimics a (very large) fetch that says "I have all major
v3 releases and want all major v4 releases." The "large" case included
X-rows as "v4.*" and Y-rows as "v3.*". This adds all release-candidate
tags to the set, which does not greatly increase the number of objects
that are considered, but does increase the number of 'from' commits,
demonstrating the quadratic nature of the previous code.

Small Case:

4fbcca4e~1: 0.85 s
  4fbcca4e: 0.26 s (num_walked: 1,011,035)
      HEAD: 0.14 s (num_walked:     8,601)

Large Case:

4fbcca4e~1: 24.0  s
  4fbcca4e:  0.12 s (num_walked:  503,925)
      HEAD:  0.06 s (num_walked:  217,243)

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