Don't replace default fetch config with branch-heads.
Don't replace default fetch config with branch-heads.

This causes the 'git config' to only add or update the branch-heads fetch
entry, and not replace any other existing entry (e.g. refs/heads/*).

Review URL: https://chromiumcodereview.appspot.com/12634013

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@189326 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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