Investigate comparison between pos and index in NSTextStorage+VimOperation.m

In [NSTextStorage xvim_firstOfLine], there is a comparison between pos and index (pos == index).

pos is a position in terms of characters but index is line the line number. It definitely feels wrong to compare them, but I was not able to figure out what the expected behavior is with my time constraints. So I'm going to leave that to those who are more familiar with the code.