Document git's ignore-revs-file feature (#1419)

* Document git's ignore-revs-file feature

A long-standing counterargument against moving to automated code formatters
like Black is that the migration will clutter up the output of git blame.
This used to be a valid argument, but not anymore. Since git version 2.23,
git natively supports ignoring revisions in blame with the --ignore-rev
and --ignore-revs-file options.

This isn't documented in the Black documentation. It should be as it would
put old projects wanting to migrate to Black at ease since blame won't
be ruined.

* Add links so people can +1 on --ignore-revs-file support

* Fix wording

'Counterargument' was the wrong word since it means an objection to an objection. 'Argument' is the proper word I was looking for.

Co-authored-by: Cooper Lees <me@cooperlees.com>