Update bundled libgit2 to >= 1.9.4 for relativeWorktrees support

[HackXIt]

Reproduction steps

Use LibGit2Sharp to open a repository/worktree created by Git with relative worktree paths enabled. Modern Git can set this automatically when a worktree is created or repaired with relative paths.

A minimal repository state has core.repositoryformatversion = 1 and the newer Git extension:

[extensions]
    relativeWorktrees = true

A linked worktree may have a .git file like:

gitdir: ../worktrees/my-worktree

Then open the worktree with LibGit2Sharp:

using LibGit2Sharp;

using var repo = new Repository("/path/to/my-worktree");

This also reproduces through downstream tools that depend on LibGit2Sharp. In my case, GitVersion.MsBuild 6.7.0 launches its bundled GitVersion tool, which bundles LibGit2Sharp 0.31.0 and native libgit2 1.8.4, and fails while opening the repository.

Expected behavior

LibGit2Sharp should open repositories/worktrees that use Git's extensions.relativeWorktrees extension.

The upstream libgit2 issue was fixed here:

• libgit2 doesn't support relative worktrees (as per git >= 2.48) libgit2#7210
• fix: Recognize relative worktrees extension libgit2#7254

The fix is included in libgit2 v1.9.4:

• https://github.com/libgit2/libgit2/releases/tag/v1.9.4

Actual behavior

Opening the repository fails before any repository operations can run:

LibGit2Sharp.LibGit2SharpException: unsupported extension name extensions.relativeworktrees
   at LibGit2Sharp.Core.Ensure.HandleError(Int32 result)
   at LibGit2Sharp.Core.Ensure.ZeroResult(Int32 result)
   at LibGit2Sharp.Core.Proxy.git_repository_open(String path)
   at LibGit2Sharp.Repository..ctor(String path, RepositoryOptions options, RepositoryRequiredParameter requiredParameter)
   at LibGit2Sharp.Repository..ctor(String path)

Version of LibGit2Sharp (release number or SHA1)

Observed via downstream GitVersion.MsBuild 6.7.0, which bundles:

LibGit2Sharp 0.31.0
native libgit2 1.8.4

The requested dependency update is for a LibGit2Sharp release that bundles native libgit2 >= 1.9.4.

Operating system(s) tested; .NET runtime tested

Tested on Linux / WSL2 with .NET 8.0.

Git version in the environment:

git version 2.51.2

Downstream dependency-chain impact

This blocks tools that consume LibGit2Sharp indirectly, notably GitVersion. I am filing a corresponding GitVersion issue as well, because GitVersion cannot fix this until LibGit2Sharp publishes a release with libgit2 >= 1.9.4.

[HackXIt]

I filed the downstream GitVersion tracking issue here: GitTools/GitVersion#4967

That issue references this one as the dependency-chain blocker: GitVersion needs a LibGit2Sharp release bundling native libgit2 >= 1.9.4 before GitVersion can update its own bundled tooling.