Fix default value of clang-format-diff's -p option.

This way, it has the same default as 'patch' and also the example in the
code makes more sense as it is explicitly setting -p 1.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182923 91177308-0d34-0410-b5e6-96231b3b80d8