fixes to `librustc_borrowck::bitslice::bits_to_string`, used for graphviz printing.
fixes to `librustc_borrowck::bitslice::bits_to_string`, used for graphviz printing.

GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust