Slight regression regarding `status_single()` on case insensitive platforms

This **[LibGit2Sharp test](https://github.com/libgit2/libgit2sharp/blob/vNext/LibGit2Sharp.Tests/StageFixture.cs#L156-L181)** doesn't pass any longer when being run against f2c4188.

The failing test has been stripped down to put this issue under the bright light of Clar.