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