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/gitdiff/gitdiff_render_test.go

@@ -17,7 +17,7 @@ import (
 
 func TestGetDiffForRender(t *testing.T) {
 	repoDir := filepath.Join(t.TempDir(), "temp-repo")
-	require.NoError(t, git.InitRepositoryLocal(t.Context(), repoDir, false, git.Sha1ObjectFormat.Name()))
+	require.NoError(t, git.InitRepositoryLocal(t.Context(), repoDir, false, git.Sha1ObjectFormat.Name(), ""))
 
 	contentLeft := strings.Repeat("a\n", 20) +
 		"mark1\n" +