cmake: Touch $GIT_DIR/logs/HEAD if it does not already exist.

Apparently some git tools (such as "repo") may not create this file.
Patch by Quentin Neill.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301565 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed