Add support for GIT_COMMON_DIR environment variable

[jelmer]

Description

Add support for the GIT_COMMON_DIR environment variable, which allows Git to use a separate git directory and work tree.

Background

The GIT_COMMON_DIR environment variable specifies the path to the git directory when it's separate from the working tree. This is particularly useful for:

• Git worktrees where multiple working directories share the same git objects
• Custom repository layouts where the .git directory is not in the working tree root

Use Case in Dulwich

This would be valuable for Dulwich users who:

• Work with multiple worktrees
• Have custom repository setups with separate git directories
• Need to interact with repositories that use this Git feature

Implementation Notes

This would require updating repository discovery and initialization code to respect the GIT_COMMON_DIR environment variable when locating git objects and references.