\\b in a pattern isn't \b

Require the start of line or any character not a slash before a \b to be
translated into vim's syntax.  Technically not 100% correct as it'll
false negative on \\\b, but that's OK for now.  I haven't run into it in
practice unlike the bug fixed by this change.
Easy reference to vim syntax for the next time I need to modify the translation
Notes on git aliases for git-viack