R1:cb033673b6eb

Unify intracluster sync and Drydock working copy construction timeouts as a repository "copy time limit"

Summary:
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…