Bug 1317594 - Vendor latest version of robustcheckout extension. a=NPOTB

File copied verbatim from changeset 249a47720ddcf896a9f07600c429a1b4492b805e
from the version-control-tools repository.

The updated extension now detects and retries after network failures
where it didn't before. This should cut down on the number of
intermittent failures.

MozReview-Commit-ID: 2bFLcGEARTJ