versioning changes

- remove VERSION_STATIC - https://community.tt-rss.org/t/versioning-changes-for-trunk/2974
- report git commit/timestamp properly by invoking git instead of trying to parse .git/HEAD etc
- remove git-related global constants used when checking for updates

include/functions.php

@@ -560,7 +560,6 @@
 				session_regenerate_id(true);
 
 				$_SESSION["uid"] = $user_id;
-				$_SESSION["version"] = VERSION_STATIC;
 				$_SESSION["auth_module"] = $auth_module;
 
 				$pdo = DB::pdo();