Add Kind::empty_blob() and Kind::empty_tree() methods to gix-hash

[Copilot]

This PR adds two new convenience methods to the Kind enum in the gix-hash crate:

• Kind::empty_blob(&self) -> ObjectId
• Kind::empty_tree(&self) -> ObjectId

These methods provide a more ergonomic way to get empty blob and tree object IDs for a specific hash kind, complementing the existing ObjectId::empty_blob(Kind) and ObjectId::empty_tree(Kind) static methods.

Usage

use gix_hash::Kind;

let kind = Kind::Sha1;

// Before: had to use ObjectId static methods
let empty_blob = ObjectId::empty_blob(kind);
let empty_tree = ObjectId::empty_tree(kind);

// Now: can use Kind instance methods directly
let empty_blob = kind.empty_blob();
let empty_tree = kind.empty_tree();

Implementation Details

• Both methods are const fn to match the existing ObjectId implementations
• They delegate to the existing ObjectId::empty_blob(kind) and ObjectId::empty_tree(kind) methods
• Follows the same pattern as the existing Kind::null() method
• Added comprehensive tests to ensure the methods return identical results to their ObjectId counterparts

This change maintains the existing gix crate's Repository::empty_blob() and Repository::empty_tree() methods while providing the requested functionality at the Kind level for consistency with the existing API design.

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