Add new test to ensure git-merge handles pull.twohead and pull.octopus

Add new test to ensure git-merge handles pull.twohead and pull.octopus

Test if the given strategies are used and test the case when multiple
strategies are configured using a space separated list.

Also test if the best strategy is picked if none is specified.  This is
done by adding a simple test case where recursive detects a rename, but
resolve does not, and verify that finally merge will pick up the
previous.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>