don't include ignored as rename candidates

I don't think that we should ever consider IGNORED as rename targets - even when we're coming with untracked for index -> workdir comparisons.

In particular, if you have some process with an exclusive lock on a file such that it could not be read by libgit2, but you have thoughtfully put that file in your `.gitignore`, then disappointingly rename detection will always fail because that file could not be read.

I suggest that it should not have ever been attempted to be read in the first place.

I don't think there's a need for a flag here to turn these on, it seems like this is *never* useful.