localrepo: Don't run case-sensitive test on macOS
macOS uses case-insensitive filesystems (HFS, APFS), so git default-enables core.ignorecase
. This causes attempts to create ref 'refs/heads/MASTER' to fail as 'refs/heads/master' already exists, and makes TestRepo_GetReferenceWithAmbiguousRefs()
fail on macOS.
Update this test to only execute the 'MASTER' case when not on macOS.