From `ArcanistUpgradeWorkflow`:
```
try {
phutil_passthru('git pull --rebase');
} catch (Exception $ex) {
phutil_passthru('git rebase --abort');
throw $ex;
}
```
Nothing's checking the return value of `phutil_passthru` (the exit status of the git pull). So even if there's an error, the "now up to date" message is still printed.