Previously, `brew update-test` is run against master branch of local
repo. However, we test PR using a detached branch in `brew test-bot`.
The result is `brew update-test` will always be up-to-date in `test-bot`.
To fix it, we create two local copies of git repo, and set master branch
to start and end sha1 correspondingly. After that, `brew update` will be
run to simulate the change between start and end sha1.
ClosesHomebrew/homebrew#43902.
Signed-off-by: Xu Cheng <xucheng@me.com>
6792847fc1