Setting default branch uses Version.slug instead of identifier

[humitos]

Details

• Read the Docs project URL: https://readthedocs.org/projects/test-builds/
• Build URL (if applicable): https://readthedocs.org/projects/test-builds/builds/8319150/
• Read the Docs username (if applicable): humitos

Expected Result

Build the branch set as default branch as when it's built as non default.

NOTE: the correct command here is git checkout --force origin/branch/with/slashes

Actual Result

The git checkout --force branch-with-slashes command fails because the branch does not exist.

---

Project.default_branch is saved in an improperly way.

>>> Version.objects.get(slug='branch-with-slashes').__dict__
...
 'type': 'branch',
 'identifier': 'origin/branch/with/slashes',
 'verbose_name': 'branch/with/slashes',
 'slug': 'branch-with-slashes',
>>> Version.objects.get(slug='branch-with-slashes').project.default_branch                                                                                                               
'branch-with-slashes'