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/generate.go

@@ -263,7 +263,7 @@ func generateRepoCommit(ctx context.Context, repo, templateRepo, generateRepo *r
 		return fmt.Errorf("readGiteaTemplateFile: %w", err)
 	}
 
-	if err = git.InitRepositoryLocal(ctx, tmpDir, false, templateRepo.ObjectFormatName); err != nil {
+	if err = git.InitRepositoryLocal(ctx, tmpDir, false, templateRepo.ObjectFormatName, ""); err != nil {
 		return err
 	}