Censor credentials with "****" instead of "xxxxx"

Summary: Ref T10982. There, and in other cases, it isn't obvious to users that we're censoring credentials when they see censored commands. Try using "****" instead of "xxxxx" to hopefully make it more clear.

Test Plan:

• Ran unit tests.
• Ran repository pull --trace, saw this output:

>>> [20] <exec> $ git clone --bare -- '********' '/Users/epriestley/dev/core/repo/local/55'
<<< [20] <exec> 3,998,331 us

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10982

Differential Revision: https://secure.phabricator.com/D16181