Make orphaned commit checks configurable

Hi there - my name's Wyatt Holden and I'm a Software Engineer at Microsoft :) I've been working with Derrick Stolee over the past few weeks on optimizing git in the Office mono-repo. We found some issues around slow checkouts on mono-repos in an automated environment:

When leaving a detached HEAD, git checkout and git switch
call orphaned_commit_warning() to warn about commits that are not reachable from any existing ref or the destination HEAD. This requires enumerating refs and walking commit history, which can be expensive in large repositories.

In an internal mono-repo, Trace2 showed tree unpacking finishing approximately 12 seconds into checkout, but the process continuing for roughly another 193 seconds. A stack dump captured the main thread inside orphaned_commit_warning(), enumerating loose refs and opening ref files. Running checkout with --quiet, which bypasses this routine, reduced the observed runtime to approximately 14 seconds. However, --quiet also suppresses unrelated output.

This three-commit series adds a targeted opt-out, advertises it when the check is slow, and enables it in Scalar:

1. checkout: make orphaned commit checks configurable Add checkout.orphanedCommitWarning , enabled by default, for both checkout and switch. Setting it to  false  skips the reachability check while retaining the previous HEAD description and normal checkout output. Add a Trace2 region to expose the check’s duration and tests verifying that disabling the setting bypasses the routine.
2. checkout: advise when orphaned commit checks are slow When the check takes more than two seconds, suggest the opt-out and explain that disabling it also removes the warning about commits left behind. Control this advice independently through advice.checkoutOrphanedCommitWarning. Test advice and suppression without relying on wall-clock delays.
3. scalar: disable orphaned commit checks by default Add checkout.orphanedCommitWarning=false to Scalar’s recommended configuration using its existing set-if-missing mechanism, preserving explicit user settings during clone, register, and reconfigure. Document the tradeoff and reflog recovery; rely on existing Scalar tests for configuration-preservation behavior.

Ordinary Git retains its existing default behavior. Users who disable the check lose the immediate warning about commits left behind; those commits can still be recovered through the HEAD reflog while the relevant entries and objects remain available.

Wyatt Holden (3):
checkout: make orphaned commit checks configurable
checkout: advise when orphaned commit checks are slow
scalar: disable orphaned commit checks by default

 Documentation/config/advice.adoc   |  6 +++
 Documentation/config/checkout.adoc | 11 +++++
 Documentation/scalar.adoc          |  7 +++
 advice.c                           |  1 +
 advice.h                           |  1 +
 builtin/checkout.c                 | 31 +++++++++++++-
 scalar.c                           |  1 +
 t/README                           |  5 +++
 t/t2020-checkout-detach.sh         | 69 ++++++++++++++++++++++++++++++
 9 files changed, 130 insertions(+), 2 deletions(-)

base-commit: 6de20f6092dcf9bdb1c8efe03db4b70c82b423dd
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2264%2Fwyatt-holden%2Fwyattholden%2Fcheckout-orphaned-commit-warning-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2264/wyatt-holden/wyattholden/checkout-orphaned-commit-warning-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/2264

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