Make gogit Repository.GetBranchNames consistent (#28348)

[GiteaBot]

Backport #28348 by @AdamMajer

nogogit GetBranchNames() lists branches sorted in reverse commit date order. On the other hand the gogit implementation doesn't apply any ordering resulting in unpredictable behaviour. In my case, the unit tests requiring particular order fail

repo_branch_test.go:24:
            Error Trace:
           ./gitea/modules/git/repo_branch_test.go:24
            Error:          elements differ

                            extra elements in list A:
                            ([]interface {}) (len=1) {
                             (string) (len=6) "master"
                            }

                            extra elements in list B:
                            ([]interface {}) (len=1) {
                             (string) (len=7) "branch1"
                            }

                            listA:
                            ([]string) (len=2) {
                             (string) (len=6) "master",
                             (string) (len=7) "branch2"
                            }

                            listB:
                            ([]string) (len=2) {
                             (string) (len=7) "branch1",
                             (string) (len=7) "branch2"
                            }
            Test:           TestRepository_GetBranches

To fix this, we sort branches based on their commit date in gogit implementation.

Fixes: #28318