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

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>