Undoing a git bisect mistake

From the git-bisect documentation:

Bisect log and bisect replay

After having marked revisions as good or bad, issue the following command to show what has been done so far:

$ git bisect log

If you discover that you made a mistake in specifying the status of a revision, you can save the output of this command to a file, edit it to remove the incorrect entries, and then issue the following commands to return to a corrected state:

$ git bisect reset
$ git bisect replay that-file

You can output a record of what was done using

$ git bisect log > bisect.log

Open that file in an editor and edit/remove the faulty line. Then you can replay it with

$ git bisect replay bisect.log

This is documented in git help bisect.