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

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 <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>