This vendors a fork of logrus with fixes to deal with errors generated
by the github.com/pkg/errors package. Without these fixes, the errors
are mishandled when passed to WithError:
- When printing to a terminal (colors enabled), stack traces are
included in the error fields on a log line.
- When printing to a file (or with colors disabled), the error value is
missing entirely.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>