Make gogit Repository.GetBranchNames consistent (#28348) (#28386)

Make gogit Repository.GetBranchNames consistent (#28348) (#28386)

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
Co-authored-by: Adam Majer <amajer@suse.de>