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.

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)