Faster navigation #86

The existing arrow navigation works for history browsing. However, viewing a commit diff takes 2 keystrokes (2 x right arrow) and 3 to enable scrolling.

It would be nice to work with the diff with less key strokes.

This can be addressed in several ways, here are a few suggestions:

• Add shortcut keys for each view, e.g. keys 1-4 for Graph, Commit, File, Diff. (1 keystroke for viewing)
• Show Graph and Diff at the same time (zero keystrokes for viewing)

See also PR #84