t3030: merge-recursive backend test.

We have fairly extensive coverage of read-tree 3-way machinery,
and many Porcelain-ish tests use git-merge front-end tests, but
we did not have good basic test for merge-recursive, which made
it very hard to hack on it.

I used this during the recent work to teach D/F conflicts to
merge-recursive.

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