Add 'ours' merge strategy.

Add 'ours' merge strategy.

This adds the coolest merge strategy ever, "ours".  It can take
arbitrary number of foreign heads and merge them into the
current branch, with the resulting tree always taken from our
branch head, hence its name.

What this means is that you can declare that the current branch
supersedes the development histories of other branches using
this merge strategy.

Signed-off-by: Junio C Hamano <junkio@cox.net>