Add support for relativeWorktrees git extension

[Copilot]

Summary

Fixes an error when using Git repositories with the relativeWorktrees extension, which is enabled when creating worktrees with relative paths.

Problem

Git supports using relative paths for worktree locations via the relativeWorktrees extension:

git worktree add --relative-paths ..\Test123 -d

However, SourceLink was throwing an error when encountering repositories with this extension:

Error reading git repository information: Unsupported repository extension 'relativeworktrees'. 
Only noop, preciousObjects, partialclone, worktreeConfig are supported.

Solution

Added "relativeWorktrees" to the list of known git extensions in GitRepository.cs. This follows the same pattern as other supported extensions (noop, preciousObjects, partialclone, worktreeConfig).

Changes

• GitRepository.cs: Added "relativeWorktrees" to the s_knownExtensions array
• GitRepositoryTests.cs: Added test OpenRepository_Version1_RelativeWorktreesExtension() to verify repositories with this extension can be opened successfully

References

• Git documentation: https://git-scm.com/docs/git-config#Documentation/git-config.txt-relativeWorktrees
• Git repository format specification: https://git-scm.com/docs/gitrepository-layout

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• st8vsblobprodcus339.vsblob.vsassets.io
  • Triggering command: /home/REDACTED/work/sourcelink/sourcelink/.dotnet/dotnet msbuild /m /nologo /clp:Summary /v:minimal /nr:true /warnaserror /p:TreatWarningsAsErrors=true /p:ContinuousIntegrationBuild=false /home/REDACTED/work/sourcelink/sourcelink/artifacts/toolset/restore.proj /t:__WriteToolsetLocation /clp:ErrorsOnly;NoSummary /p:__ToolsetLocationOutputFile=/home/REDACTED/work/sourcelink/sourcelink/artifacts/toolset/10.0.0-beta.25479.115.txt (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Error when using relative worktree paths</issue_title>
<issue_description>Git supports using relative paths for worktree locations
e.g.
git worktree add --relative-paths ..\Test123 -d

But sourcelink fails when this feature is used.

Error reading git repository information: Unsupported repository extension 'relativeworktrees'. Only noop, preciousObjects, partialclone, worktreeConfig are supported.

Please add support for the relativeWorktrees extension.
https://git-scm.com/docs/git-config#Documentation/git-config.txt-relativeWorktrees</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #1468

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