Use softtabstop instead of redefining tabstop.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61618 91177308-0d34-0410-b5e6-96231b3b80d8

utils/vim/vimrc

@@ -14,7 +14,7 @@ set nocompatible
 set textwidth=80
 
 " A tab produces a 2-space indentation
-set tabstop=2
+set softtabstop=2
 set shiftwidth=2
 set expandtab