Add is_empty_tree() and is_empty_blob() methods to gix_hash::Oid

[Copilot]

This PR implements the missing is_empty_tree() and is_empty_blob() methods for the borrowed gix_hash::oid type to match the functionality already available in the owned gix_hash::ObjectId type.

Problem

The ObjectId type already provides these convenience methods:

let empty_blob = ObjectId::empty_blob(Kind::Sha1);
assert!(empty_blob.is_empty_blob());
assert!(empty_blob.is_empty_tree() == false);

However, the borrowed oid type only had is_null() but was missing the equivalent empty blob and tree detection methods, creating an inconsistency in the API.

Solution

Added the missing methods to the oid type following the same patterns as the existing is_null() implementation:

• is_empty_blob() - Returns true if the hash equals the empty blob hash
• is_empty_tree() - Returns true if the hash equals the empty tree hash

The implementation uses the same SHA-1 byte constants as ObjectId and follows the established pattern of comparing against static helper methods.

Implementation Details

• Added static helper methods empty_blob_sha1() and empty_tree_sha1() in the "Sha1 specific methods" impl block
• Added public is_empty_blob() and is_empty_tree() methods in the "Access" impl block
• Uses identical byte constants as ObjectId::empty_blob() and ObjectId::empty_tree()
• Added comprehensive tests including consistency validation between ObjectId and oid method behavior

Now both the owned and borrowed hash types provide the same convenience methods for detecting empty Git objects.

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