Some of the backed-out changes affected a lot of files, and it's making
it slower to step backwards through ignore/annotate history.

The revisions were identified by logging all revisions referencing
"1696251", filtering by those that did backouts, then adding the backout
revisions (plus the revisions they back out) to the ignore files.

Differential Revision: https://phabricator.services.mozilla.com/D127313