merge: test the top-level merge driver

merge: test the top-level merge driver

We seem to have tests for specific merge strategy backends
(e.g. recursive), but not much test coverage for the "git merge"
itself.  As I am planning to update the semantics of merging
"FETCH_HEAD" in such a way that these two

    git pull . topic_a topic_b...

vs.

    git fetch . topic_a topic_b...
    git merge FETCH_HEAD

are truly equivalent, let me add a few test cases to cover the
tricky ones.

Signed-off-by: Junio C Hamano <redacted>