i added shortcut manually

This was not working, until i added to keymap.cson:

```
'atom-text-editor':
'alt-shift-up': 'split-selection-into-cursors:split'
```
Note I used a different shortcut than the default one because for me it's more synergic with expand-region

Start by checking the keymap.cson entry shown in the report and how the split-selection:split command is registered. Reproduce the shortcut problem in Atom, compare it with the manually added mapping, and verify that the default shortcut works without that addition.

Written by the indexing model from the issue text.