Only write Git index extensions when they contain meaningful data

Previously, dulwich would write empty extensions to the index file,
causing unnecessary bloat. Now we filter out extensions with no
data before writing.

Fixes #1643

NEWS

@@ -1,3 +1,10 @@
+0.23.2	UNRELEASED
+
+ * Only write Git index extensions when they contain meaningful data.
+   Previously, dulwich would write empty extensions to the index file,
+   causing unnecessary bloat.
+   (Jelmer Vernooij, #1643)
+
 0.23.1	2025-06-30
 
  * Support ``untracked_files="normal"`` argument to ``porcelain.status``,

dulwich/index.py

@@ -897,13 +897,21 @@ class Index:
 
         f = GitFile(self._filename, "wb")
         try:
+            # Filter out extensions with no meaningful data
+            meaningful_extensions = []
+            for ext in self._extensions:
+                # Skip extensions that have empty data
+                ext_data = ext.to_bytes()
+                if ext_data:
+                    meaningful_extensions.append(ext)
+
             if self._skip_hash:
                 # When skipHash is enabled, write the index without computing SHA1
                 write_index_dict(
                     cast(BinaryIO, f),
                     self._byname,
                     version=self._version,
-                    extensions=self._extensions,
+                    extensions=meaningful_extensions,
                 )
                 # Write 20 zero bytes instead of SHA1
                 f.write(b"\x00" * 20)
@@ -914,7 +922,7 @@ class Index:
                     cast(BinaryIO, sha1_writer),
                     self._byname,
                     version=self._version,
-                    extensions=self._extensions,
+                    extensions=meaningful_extensions,
                 )
                 sha1_writer.close()
         except: