bash 4.0 compatibility for dirdiffer.sh. In bash 4.0, "declare VAR" no longer initializes VAR if not already set.
bash 4.0 compatibility for dirdiffer.sh. In bash 4.0, "declare VAR" no longer
initializes VAR if not already set.

BUG=83180
TEST=Run dirdiffer.sh under bash 4.0 or newer (4.2.10 is current)
Review URL: http://codereview.chromium.org/7050013

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@85948 0039d316-1c4b-4281-b951-d872f2087c98

gavinp/chromium,ropik/chromium,ropik/chromium,ropik/chromium,yitian134/chromium,yitian134/chromium,gavinp/chromium,gavinp/chromium,gavinp/chromium,ropik/chromium,yitian134/chromium,adobe/chromium,adobe/chromium,adobe/chromium,yitian134/chromium,adobe/chromium,adobe/chromium,ropik/chromium,adobe/chromium,gavinp/chromium,adobe/chromium,gavinp/chromium,gavinp/chromium,gavinp/chromium,ropik/chromium,adobe/chromium,gavinp/chromium,adobe/chromium,gavinp/chromium,yitian134/chromium,yitian134/chromium,adobe/chromium,ropik/chromium,ropik/chromium,ropik/chromium,yitian134/chromium,yitian134/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium