Don't update mounted flag on React revision (#56371)

Summary:
Pull Request resolved: #56371

Changelog: [GENERAL][FIXED] - Prevent updates to mounted flag on the React revision

Currently, with fabric branching enabled, the logic for `updateMountedFlag` can run twice for the same tree: first when committing it to a react revision, then when merging to the main revision. This has the potential to drive `enableCounter_ ` negative.

This diff, makes it so that `updateMountedFlag` runs only when committing to the main branch.

Reviewed By: rubennorte, cipolleschi

Differential Revision: D99996551

fbshipit-source-id: 8e578b3bf1e22ef1f87fdd0df20dc25bb4631ccf

Author: j-piasecki

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

@@ -422,7 +422,7 @@ CommitStatus ShadowTree::tryCommit(
 
     auto newRevisionNumber = currentRevision_.number + 1;
 
-    {
+    if (!isReactBranch) {
       std::scoped_lock dispatchLock(EventEmitter::DispatchMutex());
       updateMountedFlag(
           currentRevision_.rootShadowNode->getChildren(),