lib:
- convert file index to RB tree and provide a depth-first iteration API
- use first-depth file index iteration when removing files during updates
to fix empty directories left behind, and write a test for this problem
tog:
- make diff view generate diffs in a background thread
- bug: page-down in tree view may move cursor beyond the last visible item
- implement horizonal scrolling in all views
- implement horizonal split view mode
- make it possible to move between commit diffs with a single key press
- implement search feature
