My current dev environment is destroyed in a number of interesting ways for unrelated reasons so I can't get the actual error output to show, but I just ran into this. The error happens here:
https://secure.phabricator.com/diffusion/P/browse/master/src/applications/harbormaster/step/HarbormasterLeaseWorkingCopyBuildStepImplementation.php;1a4e5931f86f49dffff6c244c5c3a8d87d1f5cb2$159
and is caused by assuming that you'll have other repositories cloned, when none are specified, you aren't returned something iterable:
https://secure.phabricator.com/diffusion/P/browse/master/src/applications/harbormaster/step/HarbormasterLeaseWorkingCopyBuildStepImplementation.php;1a4e5931f86f49dffff6c244c5c3a8d87d1f5cb2$137