Apply suggestions from code review

Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo>

src/libfetchers/git-utils.cc

@@ -1171,6 +1171,8 @@ struct GitFileSystemObjectSinkImpl : GitFileSystemObjectSink
 
     void createRegularFile(const CanonPath & path, std::function<void(CreateRegularFileSink &)> func) override
     {
+        checkInterrupt();
+
         /* Multithreaded blob writing. We read the incoming file data into memory and asynchronously write it to a Git
            blob object. However, to avoid unbounded memory usage, if the amount of data in flight exceeds a threshold,
            we switch to writing directly to a Git write stream. */
@@ -1334,6 +1336,8 @@ struct GitFileSystemObjectSinkImpl : GitFileSystemObjectSink
         auto repo(repoPool.get());
 
         [&](this const auto & visit, Directory & node) -> void {
+            checkInterrupt();
+
             // Write the child directories.
             for (auto & child : node.children)
                 if (auto dir = std::get_if<Directory>(&child.second.file))

src/libutil/include/nix/util/pool.hh

@@ -215,7 +215,7 @@ public:
     std::vector<ref<R>> clear()
     {
         auto state_(state.lock());
-        return std::move(state_->idle);
+        return std::exchange(state_->idle, {});
     }
 };