completion: work around zsh option propagation bug

When listing commands in zsh (git <TAB><TAB>), all of them will show up, instead of only porcelain ones. The root cause of this is because zsh versions from 4.3.0 to present (4.3.15) do not correctly propagate the SH_WORD_SPLIT option into the subshell in ${foo:=$(bar)} expressions. Because of this bug, the list of all commands was treated as a single word in __git_list_porcelain_commands and did not match any of the patterns that would usually cause plumbing to be excluded. With problematic versions of zsh, after running emulate sh fn () { var='one two' for v in $var; do echo $v; done } x=$(fn) : ${y=$(fn)} printing "$x" results in two lines as expected, but printing "$y" results in a single line because $var is expanded as a single word when evaluating fn to compute y. So avoid the construct, and use an explicit 'test -n "$foo" || foo=$(bar)' instead. [jn: clarified commit message, indentation style fix] Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com> Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>