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`

cmd/serv.go

@@ -195,12 +195,10 @@ func runServ(ctx context.Context, c *cli.Command) error {
 	}
 
 	if len(sshCmdArgs) < 2 {
-		if git.DefaultFeatures().SupportProcReceive {
-			// for AGit Flow
-			if cmd == "ssh_info" {
-				cprintf(c, "%s", agit.SshInfoJson)
-				return nil
-			}
+		// for AGit Flow
+		if cmd == "ssh_info" {
+			cprintf(c, "%s", agit.SshInfoJson)
+			return nil
 		}
 		return fail(ctx, "Too few arguments", "Too few arguments in cmd: %s", cmd)
 	}