HARARE — When Zimbabwe released more than 4,000 prisoners in May in a nationwide amnesty, one man with a white beard and without front teeth stood out among those vigorously celebrating as they walked out of a jail. He grinned and waved his hands in the air, and shouted the praises of Zimbabwean President Emmerson Mnangagwa, who ordered the amnesty for some offenders to ease congestion in prisons.