Update the server with current file state after a restart

Previously, ycmd would not update the LSP server with the current file
state after RestartServer subcommand had been issued. This causes some
stale diagnostics to linger.

ycmd/completers/language_server/language_server_completer.py

@@ -1236,6 +1236,9 @@ def ShutdownServer( self ):
   def _RestartServer( self, request_data, *args, **kwargs ):
     self.Shutdown()
     self._StartAndInitializeServer( request_data, *args, **kwargs )
+    self._OnInitializeComplete(
+      lambda self: self._UpdateServerWithFileContents( request_data )
+    )
 
 
   def _ServerIsInitialized( self ):