Return a '/' before names of all files through MakeFileName

Summary: // won't hurt but a missing / hurts sometimes

Test Plan: make all check; ./db_repl_stress

Reviewers: vamsi

Reviewed By: vamsi

CC: dhruba

Differential Revision: https://reviews.facebook.net/D12621

db/filename.cc

@@ -46,13 +46,10 @@ extern Status WriteStringToFileSync(Env* env, const Slice& data,
 static std::string MakeFileName(const std::string& name, uint64_t number,
                                 const char* suffix) {
   char buf[100];
-  snprintf(buf, sizeof(buf), "%06llu.%s",
+  snprintf(buf, sizeof(buf), "/%06llu.%s",
            static_cast<unsigned long long>(number),
            suffix);
-  if (name.empty()) {
-    return buf;
-  }
-  return name + "/" + buf;
+  return name + buf;
 }
 
 std::string LogFileName(const std::string& name, uint64_t number) {