`GetBranch` silently returned soft-deleted branches, contradicting
`IsBranchExist` and forcing callers to manually check `branch.IsDeleted`
everywhere.
Refactored it to `GetBranchExisting` to have a clear behavior: it only
returns the existing branch.
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Signed-off-by: Sudhanshu Singh <sudhanshuwriterblc@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>