https://www.kernel.org/pub/software/scm/git/docs/git-filter-branch.htmlRemap to ancestor
By using rev-list(1) arguments, e.g., path limiters, you can limit the set of revisions which get rewritten. However, positive refs on the command line are distinguished: we don’t let them be excluded by such limiters. For this purpose, they are instead rewritten to point at the nearest ancestor that was not excluded.