Add support for core.packedGitLimit configuration

[jelmer]

Summary

Add support for the core.packedGitLimit configuration option, which controls the maximum amount of memory that can be used to mmap pack files.

Description

The core.packedGitLimit option sets a limit on the total amount of memory used to mmap pack files. When this limit is reached, Git will unmap older pack files to make room for new ones. This helps control memory usage in repositories with many large pack files.

Benefits

• Better memory management for repositories with large pack files
• Prevents excessive memory usage in long-running processes
• Improves compatibility with Git's memory management behavior
• Essential for applications that work with very large repositories

Implementation Notes

This would involve tracking mmapped pack file memory usage and implementing an LRU-style eviction mechanism when the limit is reached.