rebase--interactive: avoid empty list in shell for-loop

The $strategy_opts variable contains a space-separated list of strategy options, each individually shell-quoted. To loop over each, we "unwrap" them by doing an eval like: eval ' for opt in '"$strategy_opts"' do ... done ' Note the quoting that means we expand $strategy_opts inline in the code to be evaluated (which is the right thing because we want the IFS-split and de-quoting). If the variable is empty, however, we ask the shell to eval the following code: for opt in do ... done without anything between "in" and "do". Most modern shells are happy to treat that like a noop, but reportedly ksh88 on AIX considers it a syntax error. So let's catch the case that the variable is empty and skip the eval altogether (since we know the loop would be a noop anyway). Reported-by: Armin Kunaschik <megabreit@googlemail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>