t5580-clone-push-unc.sh test failures

```
t5580-unc-paths.sh                               (Wstat: 256 Tests: 8 Failed: 1)
  Failed test:  6
  Non-zero exit status: 1
```

Bisect on the main repo points to this failure being added in f21d60b429 ("t5580: add Cygwin support"). That was the first time the test was run on Cygwin, so doesn't look like a regression.