Use relevant revision as base when updating mounted flag with branching enabled (#56446)

Summary:
Pull Request resolved: #56446

Changelog: [GENERAL][FIXED] Uses relevant revision as base for mounted flag updates

Similarly to the commit itself, which is already handled above, `updateMountedFlag` should be using relevant revision to compare the new tree. During react commits, it should compare to the react revision if it exists, and the main revision in all other cases.

Reviewed By: rubennorte

Differential Revision: D100800154

fbshipit-source-id: ee4f9441c598a4707db65fb6ea465245e8d8f8c1

Author: j-piasecki

packages/react-native/ReactCommon/react/renderer/mounting/ShadowTree.cpp

@@ -425,7 +425,7 @@ CommitStatus ShadowTree::tryCommit(
     {
       std::scoped_lock dispatchLock(EventEmitter::DispatchMutex());
       updateMountedFlag(
-          currentRevision_.rootShadowNode->getChildren(),
+          oldRevision.rootShadowNode->getChildren(),
           newRootShadowNode->getChildren(),
           commitOptions.source);
     }