Implement infinite Git HTTP server

This server generates a new commit every time someone pulls from it.
Features:
- Pure Go implementation of Git object format and protocols
- No dependency on git CLI commands
- Thread-safe commit generation
- Read-only Git HTTP smart protocol support
- Comprehensive test suite

Each pull creates a new commit with a unique file containing a timestamp
and counter, making the repository grow infinitely with each pull operation.

Co-Authored-By: Claude <noreply@anthropic.com>

plan.md

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+# Infinite Git HTTP Server Implementation Plan
+
+## Overview
+I'll implement a Go HTTP server that generates a new commit every time a client pulls from the repository. This creates an "infinite" Git repository where the `main` branch is updated with a new commit on each fetch operation.
+
+## Key Components
+
+### 1. HTTP Endpoints
+- `GET /info/refs?service=git-upload-pack` - Reference discovery (triggers commit generation)
+- `POST /git-upload-pack` - Handle fetch/clone with dynamically generated commits
+- Reject all push operations with 403 Forbidden
+
+### 2. Core Modules
+- `pktline` package - Handle Git's packet line format
+- `server` package - HTTP server and routing
+- `generator` package - Generate new commits on demand
+- `upload` package - Modified git-upload-pack to serve dynamic content
+- `repo` package - Git repository management
+
+### 3. Commit Generation Strategy
+- On each pull request, generate a new commit with:
+  - Timestamp in commit message
+  - Unique content (e.g., pull counter, random data, or timestamp)
+  - Parent pointing to previous HEAD of main
+- Update main branch to point to new commit
+- Use Git plumbing commands or go-git library for commit creation
+
+### 4. Project Structure
+```
+.
+├── cmd/
+│   └── infinite-git/
+│       └── main.go          # Entry point
+├── internal/
+│   ├── pktline/
+│   │   ├── reader.go        # Pkt-line reader
+│   │   └── writer.go        # Pkt-line writer
+│   ├── server/
+│   │   ├── server.go        # HTTP server
+│   │   └── handlers.go      # HTTP handlers
+│   ├── generator/
+│   │   └── commit.go        # Commit generation logic
+│   ├── upload/
+│   │   └── pack.go          # Modified git-upload-pack
+│   └── repo/
+│       └── repo.go          # Repository management
+├── go.mod
+└── README.md
+```
+
+### 5. Implementation Details
+- Initialize a bare Git repository on server startup
+- Intercept ref discovery requests to generate new commits
+- Use mutex to handle concurrent pull requests safely
+- Generate commits using either:
+  - `git hash-object`, `git update-index`, `git write-tree`, `git commit-tree`
+  - Or go-git library for pure Go implementation
+- Each commit could contain:
+  - A file with incrementing counter
+  - Timestamp of the pull request
+  - Client information (if available)
+
+### 6. Features
+- Thread-safe commit generation
+- Persistent Git repository on disk
+- Configurable commit content generation
+- Rate limiting (optional)
+- Logging of all pull operations
+- Environment variable configuration
+
+## Example Behavior
+```bash
+# First client pull
+$ git pull origin main
+# Gets commit 1a2b3c4... "Pull #1 at 2024-01-20 10:00:00"
+
+# Second client pull (moments later)
+$ git pull origin main  
+# Gets commit 5d6e7f8... "Pull #2 at 2024-01-20 10:00:05"
+```