Fix Xandikos test server: XandikosBackend → SingleUserFilesystemBackend

XandikosBackend was removed in xandikos master; the replacement is
SingleUserFilesystemBackend (same API, positional path argument).
The dulwich do_commit() incompatibility is fixed in xandikos master too.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: tobixen

tests/test_servers/embedded.py

@@ -215,7 +215,7 @@ def start(self) -> None:
             return
 
         try:
-            from xandikos.web import XandikosApp, XandikosBackend
+            from xandikos.web import SingleUserFilesystemBackend, XandikosApp
         except ImportError as e:
             raise RuntimeError("Xandikos is not installed") from e
 
@@ -228,7 +228,7 @@ def start(self) -> None:
         self.serverdir.__enter__()
 
         # Create backend and configure principal (following conf.py pattern)
-        backend = XandikosBackend(path=self.serverdir.name)
+        backend = SingleUserFilesystemBackend(self.serverdir.name)
         backend._mark_as_principal(f"/{self.username}/")
         backend.create_principal(f"/{self.username}/", create_defaults=True)