Fix support for AUTHOR presubmit check in git cl.
Fix support for AUTHOR presubmit check in git cl.

Properly load --contributor value or use the git config user.email value.

R=dpranke@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/7134097

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@88979 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools