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/files/patch.go

@@ -178,12 +178,8 @@ func ApplyDiffPatch(ctx reqctx.RequestContext, repo *repo_model.Repository, doer
 	}
 	defer t.Close()
 
-	cmdApply := gitcmd.NewCommand("apply", "--index", "--recount", "--cached", "--ignore-whitespace", "--whitespace=fix", "--binary")
-	if git.DefaultFeatures().CheckVersionAtLeast("2.32") {
-		cmdApply.AddArguments("-3")
-	}
-
-	if err := cmdApply.WithRepo(t.gitRepo).
+	if err := gitcmd.NewCommand("apply", "--index", "--recount", "--cached", "--ignore-whitespace", "--whitespace=fix", "--binary", "-3").
+		WithRepo(t.gitRepo).
 		WithStdinBytes([]byte(opts.Content)).
 		RunWithStderr(ctx); err != nil {
 		return nil, fmt.Errorf("git apply error: %w", err)