Bug 1291926 - Vendor latest robustcheckout extension. a=NPOTB

From changeset de41dae85307ad65523a7dd2e9ae788490860bdc in the
version-control-tools repository. Added without modifications.

This adds automatic retry functionality to hopefully cut down on the
number of intermittent failures related to connectivity problems.

Self reviewing because the extension changes were already peer
reviewed.

MozReview-Commit-ID: 2vMpM2QHExL