Debian unstable is shipping a broken version of libedit: it de-escapes
words before passing them to the application's tab completion function,
preventing us from recognizing backslash commands.  Fortunately,
we have enough information available to dig the original text out of
rl_line_buffer, so ignore the string argument and do that.

I view this as a temporary workaround to get the affected buildfarm
members back to green in the wake of 7c015045.  I hope we can get
rid of it once somebody fixes Debian's libedit; hence, no back-patch,
at least for now.

Discussion: https://postgr.es/m/20200103110128.GA28967@msg.df7cb.de