Censor URI credentials in CommandException summaries

Summary:
Fixes T4347. Make a simple effort to mask credentials (e.g., arising from stderr on `git fetch`) when formatting command failures for user consumption.

This isn't perfect, but I think it's a net positive overall.

Test Plan:
Added and executed unit tests. Also triggered a real exception:

  EXCEPTION (CommandException): Command failed with error #1!
  COMMAND
  echo 'http://xxxxx@example.com' && false

  STDOUT
  http://xxxxx@example.com

  STDERR
  (empty)

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4347

Differential Revision: https://secure.phabricator.com/D8546