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/links_test.go

@@ -98,6 +98,7 @@ func testLinksNoLogin(t *testing.T) {
 		assertLinkPageComplete(t, nil, link)
 	}
 	MakeRequest(t, NewRequest(t, "GET", "/.well-known/security.txt"), http.StatusOK)
+	MakeRequest(t, NewRequest(t, "GET", "/ssh_info"), http.StatusOK)
 }
 
 func testLinksRedirectsNoLogin(t *testing.T) {