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`

modules/git/config.go

@@ -73,15 +73,9 @@ func syncGitConfig(ctx context.Context) (err error) {
 		return err
 	}
 
-	if DefaultFeatures().SupportProcReceive {
-		// set support for AGit flow
-		if err := configAddNonExist(ctx, "receive.procReceiveRefs", "refs/for"); err != nil {
-			return err
-		}
-	} else {
-		if err := configUnsetAll(ctx, "receive.procReceiveRefs", "refs/for"); err != nil {
-			return err
-		}
+	// set support for AGit flow
+	if err := configAddNonExist(ctx, "receive.procReceiveRefs", "refs/for"); err != nil {
+		return err
 	}
 
 	// Due to CVE-2022-24765, git now denies access to git directories which are not owned by current user.