Documentation/git-bisect.txt: add --no-ff to merge command

The hotfix application example uses `git merge --no-commit` to apply temporary changes to the working tree during a bisect operation. In some situations this can be a fast-forward and `merge` will apply the hotfix branch's commits regardless of `--no-commit` (as documented in the `git merge` manual). In the pathological case this will make a `git bisect run` invocation loop indefinitely between the first bisect step and the fast-forwarded post-merge HEAD. Add `--no-ff` to the merge command to avoid this issue. Signed-off-by: Mihail Atanassov <m.atanassov92@gmail.com> Reviewed-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>