The recent GIT_DEPTH change introduced a [[ ]] construct
which doesn't work for zsh 5.0.2. Workaround it by tweaking the test.

The following is a demonstration to show how zsh behaves:

% if [[ "" ]];then echo hoge;fi
zsh: parse error near `]]'
% if [[ "x" ]];then echo hoge;fi
zsh: parse error near `]]'
% if [[ -n "" ]];then echo hoge;fi
% if [[ -n "x" ]];then echo hoge;fi
hoge
%

Closes-Bug: #1387943
Change-Id: Ia88de876dacb3664a7c3d8f5a035e8e50fddb678