A direct clone into an empty folder emptied the folder after any failure, which
deleted a concurrent attempt's finished clone when git had written nothing.
Clones into one folder are now mutually exclusive in-process, the cleanup is
skipped when git refused the folder, never ran, or the mount is unresponsive,
and a name linked while the clone ran is a 409 that keeps the folder.
Documents path for POST /api/cases/clone.