Disable 'Open Git Repository in Terminal' if terminals are not available (#930)

* Disable 'Open Git Repository in Terminal' if terminals are not available

https://github.com/jupyterlab/jupyterlab/blob/f0790b3ffd187b7dc923156b8ae227855abecefb/packages/terminal-extension/src/index.ts#L98

* Code formatting

Author: faucct

src/commandsAndMenu.tsx

@@ -151,7 +151,9 @@ export function addCommands(
         main.dispose();
       }
     },
-    isEnabled: () => gitModel.pathRepository !== null
+    isEnabled: () =>
+      gitModel.pathRepository !== null &&
+      app.serviceManager.terminals.isAvailable()
   });
 
   /** Add open/go to git interface command */