Add missing o2m which is referred to in web view

Fix a bug when rendering the webpage:

```
500: Internal Server Error
Error

Error message:

"'runbot.branch' object has no attribute 'builds'" while evaluating
's2h(br.builds[0].job_age)'

QWeb

Error message:

Could not evaluate expression 's2h(br.builds[0].job_age)'

The error occured while rendering the template runbot.repo and evaluating the following expression: s2h(br.builds[0].job_age)
```