ExternalProject: git clone --no-checkout causes download failure with git 2.20
An optimization introduced in git 2.20 (see fa655d8) causes a git checkout
of a branch other than the current HEAD
that follows a git clone --no-checkout
to leave the work tree unpopulated. This causes the ExternalProject
download
step to fail in CMake 3.16+ (see 627fc5b4).
Note that this does not occur with git 2.21+ (see 8424bfd).