Fix incorrect default branch when adopt a repository (#30912)
Fix #30521
we should sync branches first, then detect default branch, or
`git_model.FindBranchNames` will always return empty list, and the
detection will be wrong.
(cherry picked from commit e94723f2de7d9bf12d870f5ce9ffb291a99ba090)
Conflicts:
services/repository/adopt.go
trivial conflict because
e80466f734 Resolve lint for unused parameter and unnecessary type arguments (#30750)
was not cherry-picked
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