Error when creating a CL in a read-only checkout.
Error when creating a CL in a read-only checkout.

BUG=25824
TEST=running 'gcl change foo' in a read-only checkout
shows an error, --force creates a CL anyway, and gcl
change in read-write checkouts continues to work.
Review URL: http://codereview.chromium.org/378027

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@31452 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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