git_setup: reuse existing branches and objects
git_setup: reuse existing branches and objects

When removing origin from an existing repo, make sure we don't lose
fetched branches. Otherwise, 1) it fetches source from scratch 2) does
not delete existing objects, which leads to inflated .git size

R=iannucci@chromium.org, mmoss@chromium.org
BUG=425348

Review URL: https://codereview.chromium.org/666723004

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@292567 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build