browse: Ensure to retrieve a branch for an origin
browse: Ensure to retrieve a branch for an origin

When browsing an origin, the branch named 'HEAD' or 'master' is set as
the default one to retrieve. But some origins do not have such
branches (e.g. Debian source packages), so in that case, retrieve
the first branch found.

SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui