Import the experimental parse-tree explainer, with fixes, from the
tree-explain branch.

FossilOrigin-Name: bcbc7152d49107afa926c8950360c61a6cf3d244
diff --git a/src/main.c b/src/main.c
index 8562a90..d6dc0c7 100644
--- a/src/main.c
+++ b/src/main.c
@@ -2936,6 +2936,22 @@
       break;
     }
 
+#if defined(SQLITE_ENABLE_TREE_EXPLAIN)
+    /*   sqlite3_test_control(SQLITE_TESTCTRL_EXPLAIN_STMT,
+    **                        sqlite3_stmt*,const char**);
+    **
+    ** If compiled with SQLITE_ENABLE_TREE_EXPLAIN, each sqlite3_stmt holds
+    ** a string that describes the optimized parse tree.  This test-control
+    ** returns a pointer to that string.
+    */
+    case SQLITE_TESTCTRL_EXPLAIN_STMT: {
+      sqlite3_stmt *pStmt = va_arg(ap, sqlite3_stmt*);
+      const char **pzRet = va_arg(ap, const char**);
+      *pzRet = sqlite3VdbeExplanation((Vdbe*)pStmt);
+      break;
+    }
+#endif
+
   }
   va_end(ap);
 #endif /* SQLITE_OMIT_BUILTIN_TEST */