Add .git-blame-ignore-revs file

This new file is intended to be used to list sweeping changes
such as mass reformatting or renaming. You can configure git-blame
so that the command ignores commits listed in this file.

The initial contents of this file is two commit hashes which did
mass renaming to the lld directory.

Differential Revision: https://reviews.llvm.org/D67145

llvm-svn: 370855