givi - Allow checking out a pull-request for review

Examples:

Use upstream/4.3 as the base branch (on most repos). On the core repo, checkout the changes from pull request 417.
```
givi review upstream/4.3 https://github.com/civicrm/civicrm-core/pull/417
```

As above, but also run setup.sh after loading the code.
```
givi review upstream/4.3 https://github.com/civicrm/civicrm-core/pull/417 --setup
```

Use upstream/4.3 as the base branch (on most repos). On the core repo, checkout the changes from pull request 123. On the drupal repo, checkout the proposed changes from pull request 456. (Note: Assumes the PR was designed for Drupal 7.)
```
givi review upstream/4.3 https://github.com/civicrm/civicrm-core/pull/123 https://github.com/civicrm/civicrm-drupal/pull/456
```

As above, but assert that the PR was designed for Drupal 6.
```
givi review upstream/4.3 https://github.com/civicrm/civicrm-core/pull/123 https://github.com/civicrm/civicrm-drupal/pull/456 --d6
```