git-{merge,diff}tool refactor round three

Hopefully this is the final round =)

This series is based on top of master.

The result of this series gets us to where da/difftool in pu
arrived, but does so in a maintainer-friendly way.

I'm including Markus' patches in this series just so that it's
clear that this series is dependent on it.

Everything before "move 'git-difftool' out of contrib" can be
applied to master directly since it is not involved in the
refactor.

We'll need to revert:
	d3b8cec difftool: move 'git-difftool' out of contrib
to avoid a merge conflict in next.


In case anyone's curious, da/difftool originally
contained 25 commits.  Less is more.