Depends on D19814. Ref T13216. See PHI885. For various eldritch reasons, git fetch can hang. Although we'd probably like to fix this with git fetch --require-sustained-network-transfer-rate=512KB/5s or similar, that flag doesn't exist and we don't have a reasonable way to build it.
Short of that, move toward formalizing a repository "copy time limit": the longest amount of time anything may spend trying to make a copy of this repository.
This grows out of the existing intracluster sync limit, which is effectively the same thing. Here, apply it to git clone and git fetch in Drydock working copy construction, too. A future change may make it configurable.