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`

tests/integration/git_helper_for_declarative_test.go

@@ -169,12 +169,7 @@ func doGitCloneFail(u *url.URL) func(*testing.T) {
 func doGitInitTestRepository(dstPath string) func(*testing.T) {
 	return func(t *testing.T) {
 		// Init repository in dstPath
-		assert.NoError(t, git.InitRepositoryLocal(t.Context(), dstPath, false, git.Sha1ObjectFormat.Name()))
-		// forcibly set default branch to master
-		_, _, err := gitcmd.NewCommand("symbolic-ref", "HEAD", git.BranchPrefix+"master").
-			WithDir(dstPath).
-			RunStdString(t.Context())
-		assert.NoError(t, err)
+		assert.NoError(t, git.InitRepositoryLocal(t.Context(), dstPath, false, git.Sha1ObjectFormat.Name(), "master"))
 		assert.NoError(t, os.WriteFile(filepath.Join(dstPath, "README.md"), []byte("# Testing Repository\n\nOriginally created in: "+dstPath), 0o644))
 		assert.NoError(t, gitAddChangesDeprecated(t.Context(), dstPath, true))
 		signature := git.Signature{