Change swift-cmark repo when checking out swift-5.1-branch

[futurejones]

A recent change to swift-cmark
swiftlang/swift-cmark#15
has revealed an error in the update-checkout-config.json file.

When running update-checkout --clone --scheme swift-5.1-branch the file incorrectly checks out the master-branch for swift-cmark when it should be checking out the swift-5.1-branch

This is causing swift-5.1-branch builds to fail with this error:-

CMake Error at cmake/modules/SwiftSharedCMakeConfig.cmake:189 (include):
  include could not find load file:

    /home/worksonarm_test/jenkins_slave/workspace/swift-5.1-dev/build/buildbot_linux/cmark-linux-aarch64/src/CMarkExports.cmake

@shahmishal this is what is causing the build fails on https://ci-external.swift.org/view/swift-5.1-branch/job/oss-swift-5.1-RA-linux-ubuntu-16.04-aarch64/

[shahmishal]

Thanks!

[futurejones]

@shahmishal Thanks!
Would it be possible to add this change to the swift-5.1-branch as well?

[shahmishal]

Yes, can you cherry-pick it? I will merge it. Thanks!

[futurejones]

Added pull request #29087