Summary:
This commit introduces an `ignoreRevsFile` for use with Git blame. See
the header of the new file for details.
Test Plan:
Run `git blame -L183,183 config/test.js`. Before this change, note that
it blames to the reformatting commit. After this change, and after
configuring `git config blame.ignoreRevsFile .git-blame-ignore-revs`,
note that it blames to the last real change.
wchargin-branch: prettier-2.0.1-unblame
William Chargin