refactor(git): clarify GetBranch behavior to make it only gets an existing branch (#38662)

`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.

---------

Signed-off-by: Sudhanshu Singh <sudhanshuwriterblc@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/repository/branch_test.go

@@ -25,7 +25,7 @@ func TestSyncRepoBranches(t *testing.T) {
 	assert.NoError(t, err)
 	repo = unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
 	assert.Equal(t, "sha1", repo.ObjectFormatName)
-	branch, err := git_model.GetBranch(t.Context(), 1, "master")
+	branch, err := git_model.GetBranchExisting(t.Context(), 1, "master")
 	assert.NoError(t, err)
 	assert.Equal(t, "master", branch.Name)
 }