Choosing "Stage" or "Unstage" from context menu can do the wrong thing

[BinaryMuse]

To reproduce:

1. Select a file in the "Unstaged Changes" list
2. Right-click the empty space in the "Staged Changes" list
3. Select "Unstage"

Expected:

Nothing happens as nothing in the Staged Changes list was selected

Observed:

The selected file from the Unstaged Changes list is staged.

This happens because we dispatch core:confirm for both the "Stage" and "Unstage" context menu items.

[kuychaco]

Think it makes sense to make it such that the staging/unstaging context menu item only gets displayed when right clicking on an item