Description

Add support for the GIT_PROTOCOL environment variable, which sets the protocol version for Git operations.

Background

The GIT_PROTOCOL environment variable allows forcing a specific Git protocol version (e.g., "version=2") for network operations. This is useful for testing, debugging, or working with servers that require specific protocol versions.

Use Case in Dulwich

This would be valuable for Dulwich users who:

• Need to force protocol version 2 for better performance
• Work with servers that only support specific protocol versions
• Debug network protocol issues
• Test compatibility with different protocol versions

Implementation Notes

This would require updating the transport layer to check the GIT_PROTOCOL environment variable and use the specified protocol version when communicating with remote repositories.