consider setting `git clone --depth` in CI (Travis, AppVeyor)

[jokeyrhyme]

• https://docs.travis-ci.com/user/customizing-the-build#Git-Clone-Depth

• https://www.appveyor.com/docs/how-to/repository-shallow-clone/

• setting depth too low seems problematic, but I wonder if there is a low number that is high enough to be generally safe? 5? 10?

• Travis defaults to 50, AppVeyor seems to default to a complete clone