enhance!: raise minimum git version to 2.34 (#39565)

Raise the minimum git version to 2.34, the version in Ubuntu 22.04,
Debian 12 and RHEL 8 ship newer, and remove the fallbacks it makes
obsolete.

- Always enable AGit
- Use `diff --skip-to` and `apply -3` unconditionally
- Set the default branch of new repos and wikis via `git init
--initial-branch`
- Detect rebase conflicts via `REBASE_HEAD`

services/repository/create_test.go

@@ -24,7 +24,8 @@ func TestCreateRepositoryDirectly(t *testing.T) {
 	testRepoName := "created-repo"
 	t.Run("Success", func(t *testing.T) {
 		createdRepo, err := CreateRepositoryDirectly(t.Context(), user2, user2, CreateRepoOptions{
-			Name: testRepoName,
+			Name:          testRepoName,
+			DefaultBranch: "custom-default",
 		}, true)
 		assert.NoError(t, err)
 		assert.NotNil(t, createdRepo)
@@ -33,6 +34,10 @@ func TestCreateRepositoryDirectly(t *testing.T) {
 		assert.NoError(t, err)
 		assert.True(t, exist)
 
+		branch, err := git.GetDefaultBranch(t.Context(), createdRepo)
+		assert.NoError(t, err)
+		assert.Equal(t, "custom-default", branch)
+
 		unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{OwnerName: user2.Name, Name: createdRepo.Name})
 
 		err = DeleteRepositoryDirectly(t.Context(), createdRepo.ID)