Use boost::filesystem::current_path in ChangeToTempDir

Summary:
[Folly] Use `boost::filesystem::current_path` in `ChangeToTempDir`.

It will report failures noisily.

Reviewed By: pixelb

Differential Revision: D6493243

fbshipit-source-id: 423dc0e3a46781e9af42fee69060d31085f1a7c6

folly/experimental/TestUtil.cpp

@@ -129,14 +129,15 @@ TemporaryDirectory::~TemporaryDirectory() {
   }
 }
 
-ChangeToTempDir::ChangeToTempDir() : initialPath_(fs::current_path()) {
-  std::string p = dir_.path().string();
-  ::chdir(p.c_str());
+ChangeToTempDir::ChangeToTempDir() {
+  orig_ = fs::current_path();
+  fs::current_path(path());
 }
 
 ChangeToTempDir::~ChangeToTempDir() {
-  std::string p = initialPath_.string();
-  ::chdir(p.c_str());
+  if (!orig_.empty()) {
+    fs::current_path(orig_);
+  }
 }
 
 namespace detail {

folly/experimental/TestUtil.h

@@ -135,8 +135,8 @@ class ChangeToTempDir {
   const fs::path& path() const { return dir_.path(); }
 
  private:
-  fs::path initialPath_;
   TemporaryDirectory dir_;
+  fs::path orig_;
 };
 
 namespace detail {