Add support for core.gitProxy configuration

[jelmer]

Summary

Add support for the core.gitProxy configuration option, which specifies a proxy command for Git protocol connections.

Description

The core.gitProxy option allows users to specify a command that Git should use as a proxy for connections over the Git protocol (git://). This is useful for accessing repositories through firewalls or network configurations that require proxy access.

Benefits

• Enables access to Git repositories through corporate firewalls
• Supports network environments that require proxy connections
• Improves Git protocol compatibility in restricted networks
• Essential for enterprise and secure network environments

Implementation Notes

This would involve parsing the proxy command configuration and using it to establish Git protocol connections through the specified proxy.