I read arcanist's source (src/workflow/ArcanistWorkflow.php)
at line 271 (this isn't exactly same as current phabricator repository)
```
if ($this->useSquash) {
$this->rebase();
$this->squash();
} else {
$this->merge();
}
```
This is the reason `arc land` fails with mercurial. According to this code, I should do `arc land --squash` and It worked fine.