[PATCH] commit-graph: disable GIT_COMMIT_GRAPH_PARANOIA by default

In 7a5d604443 (commit: detect commits that exist in commit-graph but not in the ODB, 2023-10-31), we have introduced a new object existence check into `repo_parse_commit_internal()` so that we do not parse commits via the commit-graph that don't have a corresponding object in the object database. This new check of course comes with a performance penalty, which the commit put at around 30% for `git rev-list --topo-order`. But there are in fact scenarios where the performance regression is even higher. The following benchmark against linux.git with a fully-build commit-graph:

  Benchmark 1: git.v2.42.1 rev-list --count HEAD
    Time (mean ± σ):     658.0 ms ±   5.2 ms    [User: 613.5 ms, System: 44.4 ms]
    Range (min … max):   650.2 ms … 666.0 ms    10 runs

  Benchmark 2: git.v2.43.0-rc1 rev-list --count HEAD
    Time (mean ± σ):      1.333 s ±  0.019 s    [User: 1.263 s, System: 0.069 s]
    Range (min … max):    1.302 s …  1.361 s    10 runs

  Summary
    git.v2.42.1 rev-list --count HEAD ran
      2.03 ± 0.03 times faster than git.v2.43.0-rc1 rev-list --count HEAD

While it's a noble goal to ensure that results are the same regardless of whether or not we have a potentially stale commit-graph, taking twice as much time is a tough sell. Furthermore, we can generally assume that the commit-graph will be updated by git-gc(1) or git-maintenance(1) as required so that the case where the commit-graph is stale should not at all be common.

With that in mind, default-disable GIT_COMMIT_GRAPH_PARANOIA and restore the behaviour and thus performance previous to the mentioned commit. In order to not be inconsistent, also disable this behaviour by default in `lookup_commit_in_graph()`, where the object existence check has been introduced right at its inception via f559d6d45e (revision: avoid hitting packfiles when commits are in commit-graph, 2021-08-09).

This results in another speedup in commands that end up calling this function, even though it's less pronounced compared to the above benchmark. The following has been executed in linux.git with ~1.2 million references:

  Benchmark 1: GIT_COMMIT_GRAPH_PARANOIA=true git rev-list --all --no-walk=unsorted
    Time (mean ± σ):      2.947 s ±  0.003 s    [User: 2.412 s, System: 0.534 s]
    Range (min … max):    2.943 s …  2.949 s    3 runs

  Benchmark 2: GIT_COMMIT_GRAPH_PARANOIA=false git rev-list --all --no-walk=unsorted
    Time (mean ± σ):      2.724 s ±  0.030 s    [User: 2.207 s, System: 0.514 s]
    Range (min … max):    2.704 s …  2.759 s    3 runs

  Summary
    GIT_COMMIT_GRAPH_PARANOIA=false git rev-list --all --no-walk=unsorted ran
      1.08 ± 0.01 times faster than GIT_COMMIT_GRAPH_PARANOIA=true git rev-list --all --no-walk=unsorted

So whereas 7a5d604443 initially introduced the logic to start doing an object existence check in `repo_parse_commit_internal()` by default, the updated logic will now instead cause `lookup_commit_in_graph()` to stop doing the check by default. This behaviour continues to be tweakable by the user via the GIT_COMMIT_GRAPH_PARANOIA environment variable.

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 Documentation/git.txt   | 6 +++---
 commit-graph.c          | 2 +-
 commit.c                | 2 +-
 t/t5318-commit-graph.sh | 8 ++++----
 4 files changed, 9 insertions(+), 9 deletions(-)