Add sha256 object format support

[linsein]

Describe the pull request

Link to the issue: #138

ObjectFormat struct

• Add the ObjectFormat struct.
• Add the *Repository.ObjectFormat method.
• Add an ObjectFormat member to the InitOption struct

Oid interface

• Add the Oid interface to represent the Object ID of a Git repository.
• Add the SHA256 struct.
• Replace *SHA1 with Oid in other struct fields, method parameters, and return types.
• Rename the file sha1.go to oid.go.

Add SHA256 Repository testing

• Add the test_sha256 build tag to control testing variables and constants.
• Replace hard-coded hashes with the testrepoMarks map.

Others

• Fix TestReceivePack and TestUploadPack failures occurring on Git client v2.54.0 by updating the string matching patterns.

Checklist

• I agree to follow the Code of Conduct by submitting this pull request.
• I have read and acknowledge the Contributing guide.
• I have added test cases to cover the new code.

[linsein]

Just noticed that the sha256 object format was experimentally supported in Git 2.29 and marked as stable in 2.51.
If using a version of Git earlier than 2.29, users will get error: unknown option 'object-format=sha256'.

Should I give a friendlier error message? Or is it too early to add sha256 support?