Tighten and correct a few testcases for merging and cherry-picking

Tighten and correct a few testcases for merging and cherry-picking

t3501 had a testcase originally added in 05f2dfb965 (cherry-pick:
demonstrate a segmentation fault, 2016-11-26) to ensure cherry-pick
wouldn't segfault when working with a dirty file involved in a rename.
While the segfault was fixed, there was another problem this test
demonstrated: namely, that git would overwrite a dirty file involved in a
rename.  Further, the test encoded a "successful merge" and overwriting of
this file as correct behavior.  Modify the test so that it would still
catch the segfault, but to require the correct behavior.  Mark it as
test_expect_failure for now too, since this second bug is not yet fixed.

t7607 had a test added in 30fd3a5425 (merge overwrites unstaged changes in
renamed file, 2012-04-15) specific to looking for a merge overwriting a
dirty file involved in a rename, but it too actually encoded what I would
term incorrect behavior: it expected the merge to succeed.  Fix that, and
add a few more checks to make sure that the merge really does produce the
expected results.

Reviewed-By: Stefan Beller <sbeller@google.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>