Add git-blame-ignore-revs file

Add git-blame-ignore-revs file

This is helpful when using git-blame on files that 'black' heavily
modified back in 2019'ish. While git doesn't specify this name, there
seems to be a general convention around using this. See Django [1] and
Twisted [2], for example.

[1] https://github.com/django/django/blob/main/.git-blame-ignore-revs
[2] https://github.com/twisted/twisted/blob/trunk/.git-blame-ignore-revs

Signed-off-by: Stephen Finucane <stephen@that.guru>
Change-Id: If77288bfebd110fb6aa1c07e7ba2ca33c2fb40cb