bug#20901: Can't replace regexps with slash

[Paul Eggert]

replace-regexp currently mishandles regular expression replacement with 
characters like '/'.  I assume this is related to the recent changes with string 
search so I am CC'ing Artur.

This is with Emacs master (commit ff80437668dd0d5163c9e43f0c200b923b39166b).

To reproduce, run emacs -Q and type:

C-h t M-x replace-regexp RET .*/ RET RET

Emacs will respond "Replaced 0 occurrences" without changing the buffer.  It 
should replace 12 occurrences.