Don't error on first run of arc diff

Summary:
When running arc diff in a repository that supports commit ranges, it is
possible that the setting for the default relative commit hasn't been set.
If this is the case, the user will be prompted. This change makes sure that
the prompt happens (and thus the setting is set) before we run the
background lint and unit runs.

Test Plan:
```
rm .git/arc/default-relative-commit
arc diff
```

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T2351

Differential Revision: https://secure.phabricator.com/D6854