go/libraries/doltcore/sqle/dsess: transactions.go: When merging working sets on call dolt_commit, ensure new commit also gets the merged commit root value.

go/libraries/doltcore/sqle/dsess/transactions.go

@@ -232,6 +232,9 @@ func (tx *DoltTransaction) doCommit(
 			}
 
 			var newCommit *doltdb.Commit
+			if commit != nil {
+				commit.Roots.Staged = mergedWorkingSet.WorkingRoot()
+			}
 			mergedWorkingSet, newCommit, err = writeFn(ctx, tx, commit, mergedWorkingSet, existingWSHash)
 			if err == datas.ErrOptimisticLockFailed {
 				// this is effectively a `continue` in the loop

go/libraries/doltcore/sqle/enginetest/dolt_transaction_commit_test.go

@@ -272,6 +272,13 @@ func TestDoltTransactionCommitTwoClients(t *testing.T) {
 				Query:    "/* client c */ SELECT * FROM x ORDER BY y;",
 				Expected: []sql.Row{{1, 1}, {2, 2}, {3, 3}},
 			},
+			// After we commit both of these transactions, our working set should not have any pending changes.
+			// In the past, we have merged the working set but failed to land the merged root value in the
+			// commit itself.
+			{
+				Query:    "/* client c */ SELECT COUNT(*) FROM DOLT_DIFF('HEAD', 'WORKING', 'x');",
+				Expected: []sql.Row{{0}},
+			},
 		},
 	})
 	_, err := harness.NewEngine(t)