Bug 1389715 - Vendor latest robustcheckout. r=ted, a=release

From revision 19db5f4b5b10f639d24e69a4f21e4e80c6e5bbdd of
version-control-tools. Previous revision was
249a47720ddcf896a9f07600c429a1b4492b805e.

Changes include:

• Use new vfs APIs when available (bug 1359959)
• Mark as compatible with Mercurial 4.2 (bug 1361182)
• Retry after SSLError (bug 1354824)
• Mark as compatible with Mercurial 4.3
• Detect and recover from open locks (bug 1297153)

The most significant is the last one. TaskCluster can SIGKILL
hg, leading to orphaned transactions and locks. This can lead
to timeouts or use of a corrupt repository in some scenarios.
Those problems should no longer occur.

MozReview-Commit-ID: QAOSLyc0xD