Fix the inverse tilde substitution in my X window titles.

It had two problems. Firstly, on bash 4.2 systems it converted
/home/me into \~ rather than just ~, because the backslash I added to
stop the ~ in the replacement text being expanded by bash 4.3 comes
out as literal in 4.2. Secondly, it wasn't prefix-safe - if there was
a /home/simonxxx then it would substitute it as ~xxx.


git-svn-id: svn://svn.tartarus.org/sgt@10189 cda61777-01e9-0310-a592-d414129be87e