Fix test failure in aborting an index update

Apparently the value we return is passed all the way up, unmodified. We have to return `GIT_EUSER` in order for it to arrive that way at the original call point.

Fixes [a test failure](https://github.com/libgit2/objective-git/blob/84d196cbb41c71c69011826939e35e7afb210a97/ObjectiveGitTests/GTIndexSpec.m#L133) from #306.

@PiersonBro @dannygreg