validation: fix issue with an interrupted -reindex

If a reindex was interrupted while it was iterating
through the block files, genesis will already be connected
when the reindex resumes at the next startup.
In this case, a call to ActivateBestChainState() is not only unnecessary,
but it would connect multiple blocks without applying
-assumevalid, which is much slower.
This is because assumevalid requires us to have a header above
the minimum chainwork, but that header is unknown to us if it's in
a later blockfile not indexed yet.

src/validation.cpp

@@ -5157,7 +5157,13 @@ void ChainstateManager::LoadExternalBlockFile(
                 }
 
                 // Activate the genesis block so normal node progress can continue
-                if (hash == params.GetConsensus().hashGenesisBlock) {
+                // During first -reindex, this will only connect Genesis since
+                // ActivateBestChain only connects blocks which are in the block tree db,
+                // which only contains blocks whose parents are in it.
+                // But do this only if genesis isn't activated yet, to avoid connecting many blocks
+                // without assumevalid in the case of a continuation of a reindex that
+                // was interrupted by the user.
+                if (hash == params.GetConsensus().hashGenesisBlock && WITH_LOCK(::cs_main, return ActiveHeight()) == -1) {
                     BlockValidationState state;
                     if (!ActiveChainstate().ActivateBestChain(state, nullptr)) {
                         break;