When running:
$ localectl set-locale LC_MESSAGES=<TAB>
One is prompted with a list of locale fields instead of the list of
valid locales. This is because by calling "compset -P1 '*='", we modify
the $PREFIX special parameter before testing whether it contains an
equal sign. Therefore
if [[ -prefix 1 *\= ]]
is always false, and we always suggest a list of locale fields to the
user.
Fixes: #27955
3c6fefd879