Move get_chunker to __init__.py and update Chunker signature

Relocated `get_chunker` function from `chunker` module to `chunkers.__init__.py` for improved organization. Updated `Chunker` class signature to include a `sparse` parameter with a default value. Adjusted imports and type hints accordingly.

src/borg/chunkers/__init__.py

@@ -1,2 +1,15 @@
 from .chunker import *  # noqa
 from .reader import *  # noqa
+
+
+def get_chunker(algo, *params, **kw):
+    if algo == "buzhash":
+        seed = kw["seed"]
+        sparse = kw["sparse"]
+        return Chunker(seed, *params, sparse=sparse)
+    if algo == "fixed":
+        sparse = kw["sparse"]
+        return ChunkerFixed(*params, sparse=sparse)
+    if algo == "fail":
+        return ChunkerFailing(*params)
+    raise TypeError("unsupported chunker algo %r" % algo)

src/borg/chunkers/chunker.pyi

@@ -1,4 +1,4 @@
-from typing import List, Any, Iterator, BinaryIO
+from typing import List, Iterator, BinaryIO
 
 from .reader import fmap_entry
 
@@ -6,7 +6,6 @@ API_VERSION: str
 
 def buzhash(data: bytes, seed: int) -> int: ...
 def buzhash_update(sum: int, remove: int, add: int, len: int, seed: int) -> int: ...
-def get_chunker(algo: str, *params, **kw) -> Any: ...
 
 class ChunkerFailing:
     def __init__(self, block_size: int, map: str) -> None: ...
@@ -18,6 +17,12 @@ class ChunkerFixed:
 
 class Chunker:
     def __init__(
-        self, seed: int, chunk_min_exp: int, chunk_max_exp: int, hash_mask_bits: int, hash_window_size: int
+        self,
+        seed: int,
+        chunk_min_exp: int,
+        chunk_max_exp: int,
+        hash_mask_bits: int,
+        hash_window_size: int,
+        sparse: bool = False,
     ) -> None: ...
     def chunkify(self, fd: BinaryIO = None, fh: int = -1, fmap: List[fmap_entry] = None) -> Iterator: ...

src/borg/chunkers/chunker.pyx

@@ -444,16 +444,3 @@ def buzhash_update(uint32_t sum, unsigned char remove, unsigned char add, size_t
     sum = _buzhash_update(sum, remove, add, len, table)
     free(table)
     return sum
-
-
-def get_chunker(algo, *params, **kw):
-    if algo == 'buzhash':
-        seed = kw['seed']
-        sparse = kw['sparse']
-        return Chunker(seed, *params, sparse=sparse)
-    if algo == 'fixed':
-        sparse = kw['sparse']
-        return ChunkerFixed(*params, sparse=sparse)
-    if algo == 'fail':
-        return ChunkerFailing(*params)
-    raise TypeError('unsupported chunker algo %r' % algo)