make ROOTTREE default storage for metadata instead of ROOTTREEINDEX

Follow-up from !67839 (merged), in particular !67839 (comment 7448149).

Bonus: fix a bug in xAOD::Utils::getFirstBranchMatch (909bbd78, e34b7405), see: !67972 (comment 7449441)

Control/xAODRootAccess/Root/Utils.cxx

@@ -320,7 +320,7 @@ namespace xAOD {
          const TObjArray * pBranches = tree->GetListOfBranches();
          const std::regex pattern( ".*" + pre + ".*" );
 
-         for( int i = 0; i <= pBranches->GetLast() ; ++i ) {
+         for( int i = 0, nLast = pBranches->GetLast(); i <= nLast; ++i ) {
             const std::string name = pBranches->At(i)->GetName();
 
             if( std::regex_match( name, pattern ) )