[LS] [AI Context] Local Git Diff Provider - Language Server

Please see the epic for the product requirements.

Problem to Solve

We need to enable users to add git diffs as context in Duo Chat to allow them to ask questions about their local changes. While we initially considered using isomorphic-git for handling git operations, performance testing against the GitLab monolith repository showed significant performance issues that would impact user experience.

Proposed Solution

Implement a Language Server-based git diff provider that will:

1. Create a custom RPC request channel between the Language Server and editor clients (like VSCode) for fetching git diffs
2. Determine the main/default branch deterministically using the following priority order:
   • Check remote HEAD reference file (.git/refs/remotes/<remote>/HEAD)
   • Query remote repository info if reference file not available
   • Look for common branch names in local branches (main, master, trunk, development)
   • Default to "main" if no other method succeeds
3. Expose git diff options to the user:
   • Diff from main branch (default)
   • Diff from HEAD (working state)
   • Diff from any other local branch

Note: Fetching commit content is out of scope for this issue and will be handled separately.

API Flow

sequenceDiagram
    participant DC as DuoChat UI
    participant LS as Language Server
    participant VS as VSCode Extension
    participant Git as Git (VSCode API)

    DC->>LS: User selects git diff context
    LS->>VS: Request: $/gitlab/ai-context/git-diff
    Note over LS,VS: Contains repositoryUri and optional branch
    VS->>Git: getDiffWithHead() or getDiffWithBranch()
    Git-->>VS: Diff content
    VS-->>LS: Diff string
    LS-->>DC: Context item with diff content

Technical Details

• The Language Server will implement a LocalGitContextProvider that handles git context operations
• Instead of using isomorphic-git for diff operations, we'll rely on editor-specific git implementations via RPC
• The VSCode extension will implement the RPC endpoint using the built-in Git extension API
• Diffs will be fetched on-demand when context items are added to chat

Main Branch Resolution Logic

Based on the implementation in the diff, the provider will resolve the main branch in this order:

1. Check .git/refs/remotes/<remote>/HEAD for the default branch reference
2. If not found, attempt to get remote repository info (for public repos)
3. If remote info unavailable, check local branches for common names in priority order:
   • main
   • master
   • trunk
   • development
4. Default to "main" if no other method succeeds

This implementation allows for reliable branch resolution while avoiding the performance issues encountered with isomorphic-git.