`magit-quote-curly-braces': fix case-insensitive matching

To make searching and matching case-insensitive, we have to set
`case-fold-search' (as opposed to `case-fold', which is a
non-existent variable) to t.  I'm guessing this went unnoticed
because `case-fold-search' is set to t by default.

Signed-off-by: Pieter Praet <pieter@praet.org>