Speed up git-llvm script by only svn up'ing affected directories.

Also, support modifications to toplevel files in git (which need to be
committed to "monorepo-root" in svn).

Differential Revision: https://reviews.llvm.org/D54341

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@347103 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed