Replace std::cin.get with getchar. Fix #334

To avoid C++ iostream headache:
std::cin has a bad() state
std::{cin,cout} are also tied by default, reading from cin will cause a flush of cout, which may deadlock

src/command_line.cc

@@ -32,7 +32,7 @@
 #include <rapidjson/error/en.h>
 #include <loguru.hpp>
 
-#include <climits>
+#include <stdio.h>
 #include <functional>
 #include <iostream>
 #include <iterator>
@@ -478,7 +478,7 @@ int main(int argc, char** argv) {
 
   if (HasOption(options, "--wait-for-input")) {
     std::cerr << std::endl << "[Enter] to exit" << std::endl;
-    std::cin.get();
+    getchar();
   }
 
   if (print_help) {

src/language_server_api.cc

@@ -5,6 +5,7 @@
 #include <doctest/doctest.h>
 #include <loguru.hpp>
 
+#include <stdio.h>
 #include <iostream>
 
 MessageRegistry* MessageRegistry::instance_ = nullptr;
@@ -107,8 +108,8 @@ TEST_SUITE("FindIncludeLine") {
 }
 
 optional<char> ReadCharFromStdinBlocking() {
-  char c;
-  if (std::cin.read(&c, 1))
+  int c = getchar();
+  if (c >= 0)
     return c;
   return nullopt;
 }

src/test.cc

@@ -5,6 +5,7 @@
 #include "serializer.h"
 #include "utils.h"
 
+#include <stdio.h>
 #include <iostream>
 
 void Write(const std::vector<std::string>& strs) {
@@ -96,8 +97,8 @@ std::string FindExpectedOutputForFilename(
   }
 
   std::cerr << "Couldn't find expected output for " << filename << std::endl;
-  std::cin.get();
-  std::cin.get();
+  getchar();
+  getchar();
   return "{}";
 }
 
@@ -243,8 +244,8 @@ bool RunIndexTests(const std::string& filter_path, bool enable_update) {
               << "[Enter to continue - type u to update test, a to update all]";
           char c = 'u';
           if (!update_all) {
-            c = (char)std::cin.get();
-            std::cin.get();
+            c = getchar();
+            getchar();
           }
 
           if (c == 'a')