Staging current file - keyboard shortcut when focus elsewhere

[davidfraser]

When viewing local changes, I am often looking at the file whose changes I want to commit, click on the commit message and type it, and then press Ctrl/Cmd-Enter to commit, only to receive the indication I haven't staged anything.
What I'd like to be able to do is just stage the changes to the currently open file. I can do this if I've just clicked on the filename by pressing enter/space, since the focus is on the file tree, and I can stage individual changes if the focus is on the file view using Ctrl/Cmd-S, but if the focus isn't on the file tree, I have to use the mouse to click on it before pressing Enter to stage those changes
Is there a keyboard shortcut to stage the current file, when focus is on the commit message? I would be happy to add a custom command with shortcut but not sure if that's possible

PS just discovered sourcegit and absolutely loving it, best git gui I've found!