Inherit default branch name for subgroups

What does this MR do?

Contributes to #293953

Problem

We fetch default branch name from the direct project's group. But we ignore default branch settings from the root group when the project's group is a subgroup.

Solution

Use root group default branch name settings if the direct project's group doesn't redefine them.

Screenshots (strongly suggested)

Root group branch name	Project group branch name	Screenshot
main	nil
main	subgroup
nil	nil

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