Error out in `gclient config` if --name is relative
Error out in `gclient config` if --name is relative

This helps prevent behavior such as setting '--name .' so that the .gclient
file contains '.' as the name of the solution, and the .gclient file ends
up a sibling of the .git file, rather than a sibling of the git checkout's
containing directory.

R=iannucci@chromium.org, thestig@chromium.org

Review URL: https://codereview.chromium.org/1406053003

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@297343 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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