Share transient libgit2 config lookup helper

[tyrielv]

Note

This PR is now a single commit (5471e46), squashed for a clean review/merge history.

Summary

• add LibGit2Repo.GetConfigBoolOrDefault(...) (instance + static overloads) for one-off boolean config reads, replacing scattered short-lived LibGit2Repo/LibGit2RepoInvoker usage at 4 call sites
• avoid LibGit2RepoInvoker's object-store preload for reads that only need a single config value and immediately dispose the repo
• cover happy-path, unset-key, and exception-fallback behavior with unit tests

Motivation

LibGit2RepoInvoker.InitializeSharedRepo() intentionally forces an object-store probe so long-lived/shared callers can amortize object-store load costs. That is wasted work for one-off config reads that immediately dispose the repo. This change centralizes the short-lived config-read pattern behind two methods.

Design

The helper methods live directly on LibGit2Repo rather than a separate extension class — this matches the repo.GetConfigBoolOrDefault(name, default) convention already documented in AGENTS.md, and avoids unnecessary indirection for a class the team owns in the same assembly:

• public bool GetConfigBoolOrDefault(string key, bool defaultValue) — instance method, reads from an already-open repo using this.Tracer.
• public static bool GetConfigBoolOrDefault(ITracer tracer, string repoPath, string key, bool defaultValue) — opens a transient LibGit2Repo, delegates to the instance method, and disposes it.

Both fall back to defaultValue and log a RelatedWarning on any failure (matching the "default on any failure" contract each call site previously implemented independently).

Also added a protected LibGit2Repo(ITracer tracer) constructor purely to support test doubles that need to inject a mock tracer without opening a real repo.

Refactored call sites

• GVFS/GVFS/CommandLine/CloneVerb.cs — gvfs.trust-pack-indexes (only read when clone succeeded)
• GVFS/GVFS.Hooks/Program.cs — gvfs.show-hydration-status (runs on every git hook invocation)
• GVFS/GVFS.Mount/InProcessMount.cs — gvfs.background-cache-auth
• GVFS/GVFS/CommandLine/PrefetchVerb.cs — gvfs.prefetch-offload
• GVFS/GVFS.Hooks/GVFS.Hooks.csproj — updated to drop the now-removed extension-class source file

Survey notes / intentionally left alone

Surveyed master for short-lived config-only LibGit2Repo/LibGit2RepoInvoker usage. These were left alone because they are shared/long-lived repo access, not the transient anti-pattern this PR addresses:

• GVFS/GVFS.Common/Maintenance/PrefetchStep.cs
• GVFS/GVFS.Common/GitStatusCache.cs
• GVFS/GVFS.Common/Git/GitRepo.cs

Stacked PR

#2065 ("Fix NullReferenceException when cloning into a non-empty directory") is rebased on top of this branch and depends on the LibGit2Repo.GetConfigBoolOrDefault helper added here. This PR should merge first.

Review

Reviewed with an internal 6-lens review-swarm pass (correctness, security, design, tests, async-parallelism, risk-rollout). All actionable findings were addressed:

• Widened the shared helper's exception handling back to a plain catch (Exception), restoring the "default on any failure" guarantee InProcessMount/PrefetchVerb relied on before this refactor.
• Fixed a double-RelatedWarning log on the repo-open-failure path.
• Replaced a hardcoded, non-portable Z:\... path in a unit test with a GUID-suffixed temp path.
• Added missing test coverage for the unset-key (null-coalescing) branch and the InvalidDataException catch arm.
• Simplified the parameterless constructor to delegate to the tracer-accepting one.

Validation

• dotnet build GVFS\GVFS.UnitTests\GVFS.UnitTests.csproj -c Debug
• out\GVFS.UnitTests\bin\Debug\net10.0-windows10.0.17763.0\win-x64\GVFS.UnitTests.exe
  • Result: 891 passed, 0 failed, 11 skipped (pre-existing, unrelated)