Update .git/objects/info/alternates when upstream url changes.
Update .git/objects/info/alternates when upstream url changes.

BUG=497943
R=hinoka@chromium.org,agable@chromium.org

Review URL: https://codereview.chromium.org/1163403003

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@295568 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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