The test's `rm -rf "$NVM_DIR"` intermittently failed with
`rm: cannot remove...: Directory not empty`
and aborted under `set -e`.
Cause: after the clone/fetch, git can spawn a detached background gc/maintenance process that keeps writing into the clone dir while `rm -rf` runs (a concurrent writer makes the final rmdir fail).
It is not egress- or version-related
(it reproduces with all endpoints allowed),
and it is environment-timing dependent
(recently became consistent on the GitHub runners).
Disable git's background work for the test (gc.autoDetach / maintenance.auto)
so all git operations finish synchronously, and retry the rm once as a safety net.
`[ "${head_ref}" != "${avoid_ref}"]` is missing the space before the closing bracket,
so the shell prints `[: missing ']'` and the avoid_ref assertion never actually runs
(it is inside an `if` condition, so the error was non-fatal and silently disabled the check)
