fix(svn): fixed txn_id extraction from the data buffer.

- This buffer is always BYTES
- use new python3 getvalue that can get the data and not exhaust the iterator
- fixes # 5707

rhodecode/lib/middleware/simplevcs.py

@@ -599,13 +599,13 @@ class SimpleVCS(object):
         also handles the locking exceptions which will be triggered when
         the first chunk is produced by the underlying WSGI application.
         """
+
         txn_id = ''
         if 'CONTENT_LENGTH' in environ and environ['REQUEST_METHOD'] == 'MERGE':
             # case for SVN, we want to re-use the callback daemon port
             # so we use the txn_id, for this we peek the body, and still save
             # it as wsgi.input
-            data = environ['wsgi.input'].read()
-            environ['wsgi.input'] = io.StringIO(data)
+            data: str = safe_str(environ['wsgi.input'].getvalue())
             txn_id = extract_svn_txn_id(self.acl_repo_name, data)
 
         callback_daemon, extras = self._prepare_callback_daemon(