I fixed this by installing a newer version of Git. The version I installed is 2.10.2 from https://git-scm.com. See the last post here: https://www.bountysource.com/issues/31602800-git-fails-to-authenticate-access-to-private-repository-over-https
With newer Git Bash, the credential manager window pops up and you can enter your username and password, and it works!