Merge branch 'kn/refs-files-case-insensitive' into maint-2.51

Deal more gracefully with directory / file conflicts when the files backend is used for ref storage, by failing only the ones that are involved in the conflict while allowing others. * kn/refs-files-case-insensitive: refs/files: handle D/F conflicts during locking refs/files: handle F/D conflicts in case-insensitive FS refs/files: use correct error type when lock exists refs/files: catch conflicts on case-insensitive file-systems