Make Git prompt for passwords when the user provides a username but not a password

Summary: Fixes T10797. This seems to fix things on my local system.

Test Plan:

• Cloned with a username, got prompted for a password.
• Cloned with a username + password.
• Cloned with a username + bad password (error).

Reviewers: chad

Reviewed By: chad

Subscribers: Grimeh

Maniphest Tasks: T10797

Differential Revision: https://secure.phabricator.com/D15706