completion: work around zsh option propagation bug

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>