Keybinding adjustments and debugging default improvements

[yves-biener]

This PR addresses my main two issues with this otherwise awesome project.

1. The creation of a debug.log file when running diffnav serves no purpose for the end-user, as this is pretty early software, I can see the need. Hence it is now united with the tea debug log using the environment variable check too.
2. I'm too used to scroll pagers using j and k (and others too - see Change jk keybinds or make them configurable #12), hence I changed the keybindings to the following:
   t: Toggle file tree
   /: Search
   J: Down in file tree
   K: Up in file tree
   j,k,ctrl+d,ctrl+u support for embedded delta view (viewport)

[dlvhdr]

Thanks for the PR @yves-biener!
I think this is a matter of what you're used to, I personally don't really like J/K for navigation. I think introducing a config file for overriding keys is the best approach here, as everyone has their own preferences.
Who knows, maybe I'll switch someday to your method of navigation :)

Another option I considered is having a notion of "focused widget" where j/k control the focused widget, whether it's the file tree or diff viewer.

The debug.log changes are nice, we can probably merge those. If you could split it to its own PR, that would make it easier.

[dlvhdr]

I've fixed the debugging mistake in #34