Sort .git-blame-ignore-revs by commit order

This makes it easier to add blocks of commits such as
4ca5df80393e1f2..c90d0600ee000e78 which do a single thing over the
course of multiple commits. Sorting by hash would require manually
inserting each one with a comment.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D42601