ExternalProject: Avoid unnecessary checkout on clone
ExternalProject_Add checks out the HEAD, and then immediately overwrites it with the desired
This requires extra time, to check out a (potentially large) HEAD, extra bandwidth to download it, and is a potential failure point, if the HEAD has issues. For example, a particularly long file path that goes over the OS's limit (Thank you Windows 7).
Tell git not to perform a checkout on clone, since the very next command is to perform a checkout of the desired target. This may also help with Issue #17770.
To the best of my research, this should have no negative side-effects, but if someone more familiar with git says it will, I would trust them. As evidenced by the way I messed up the last merge request.