flags: introduce explicit testing for merging change to exec flag 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. Differential Revision: https://phab.mercurial-scm.org/D8529