Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/mocks/ReentrancyMock.sol

@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../security/ReentrancyGuard.sol";
+import "./ReentrancyAttack.sol";
+
+contract ReentrancyMock is ReentrancyGuard {
+    uint256 public counter;
+
+    constructor() {
+        counter = 0;
+    }
+
+    function callback() external nonReentrant {
+        _count();
+    }
+
+    function countLocalRecursive(uint256 n) public nonReentrant {
+        if (n > 0) {
+            _count();
+            countLocalRecursive(n - 1);
+        }
+    }
+
+    function countThisRecursive(uint256 n) public nonReentrant {
+        if (n > 0) {
+            _count();
+            (bool success, ) = address(this).call(abi.encodeWithSignature("countThisRecursive(uint256)", n - 1));
+            require(success, "ReentrancyMock: failed call");
+        }
+    }
+
+    function countAndCall(ReentrancyAttack attacker) public nonReentrant {
+        _count();
+        bytes4 func = bytes4(keccak256("callback()"));
+        attacker.callSender(func);
+    }
+
+    function _count() private {
+        counter += 1;
+    }
+
+    function guardedCheckEntered() public nonReentrant {
+        require(_reentrancyGuardEntered());
+    }
+
+    function unguardedCheckNotEntered() public view {
+        require(!_reentrancyGuardEntered());
+    }
+}