From: Joe Korty <joe.korty@ccur.com>

This patch replaces the call to bitmap_shift_right() in bitmap_parse() with
bitmap_shift_left().

I also prepended comments to the bitmap_shift_* functions defining what
'left' and 'right' means.  This is under the theory that if I and all the
reviewers were bamboozled, others in the future occasionally might be too.