feature request: select next file when staging chunks

[Danielku15]

Current behavior:

When staging files as a whole, SourceGit automatically selects the next file to continue staging. This happens when double-clicking or using the stage button in the toolbar.

When staging individual "chunks" in the code-diff, and the last chunk is staged, you end up with an empty selection and you see "select file to view changes".

Requested improvement:

After staging the last chunk, SourceGit should also select the next file so you can continue staging.