Make git-recursive the default strategy for git-pull.

Make git-recursive the default strategy for git-pull.

This does two things:

 - It changes the hardcoded default merge strategy for two-head
   git-pull from resolve to recursive.

 - .git/config file acquires two configuration items.
   pull.twohead names the strategy for two-head case, and
   pull.octopus names the strategy for octopus merge.

IOW you are paranoid, you can have the following lines in your
.git/config file and keep using git-merge-resolve when pulling
one remote:

	[pull]
		twohead = resolve

OTOH, you can say this:

	[pull]
		twohead = resolve
		twohead = recursive

to try quicker resolve first, and when it fails, fall back to
recursive.

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