It turns out that we do not seems to test the simple case for merging exec flag
changes. More advanced case are test (merging exec flag without a common
ancestors, merging with a symlink, etc…) but not the basic.
We are about introduce various fixes to merging flag change across renames,
having the most basic case tested first seems useful.
note: We are only testing "adding" an exec flag here, not removing it. We
introduce basic test on stable and will consolidate them on default.