Completely disable git/svn version checking if not needed.

Working with git on a branch I find it really annoying that committing
a change causes ninja to think that stuff needs to be rebuilt.

With this change at least nothing in llvm needs to be rebuild when
something is committed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306858 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed