checkout/restore: add basic tests for --merge

checkout/restore: add basic tests for --merge

Even though "checkout --merge -- paths" had some tests, we never
made sure it worked to recreate the conflicted state _after_ the
resolution was recorded in the index.  Also "restore --merge" did
not even have any tests.

Currently these commands use the unmerge_marked_index() interface
that cannot handle paths that have been resolved as removal, and
tests for that case are marked with test_expect_failure; these
should eventually be fixed, but not in this patch.

Signed-off-by: Junio C Hamano <gitster@pobox.com>