Let `gs_diff` key bindings act like a toggle
Let `gs_diff` key bindings act like a toggle

We implemented this for "inline-diff" views, let the diff views follow.

The idea is to setup one key binding, for example

```
  {
      "keys": ["ctrl+shift+."],
      "command": "gs_diff",
      "args": { "current_file": true }
  },
```

which will then either create or switch to a diff view *or* close the
diff view if it's already active.

divmain/GitSavvy,divmain/GitSavvy,divmain/GitSavvy