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.

llvm-svn: 306858
GitOrigin-RevId: cd6d9f3e5aba061390a90505e109fc65fb8dea31
3 files changed