Fix gc_picker (#144) (#146)

Signed-off-by: liumh8 <571621369@qq.com>

Fixes #144 

src/blob_gc_picker.cc

@@ -66,7 +66,9 @@ std::unique_ptr<BlobGC> BasicBlobGCPicker::PickBlobGC(
                   "got batch size %" PRIu64 ", estimate output %" PRIu64
                   " bytes",
                   batch_size, estimate_output_size);
-  if (blob_files.empty() || batch_size < cf_options_.min_gc_batch_size) {
+  if (blob_files.empty() ||
+      (batch_size < cf_options_.min_gc_batch_size &&
+       estimate_output_size < cf_options_.blob_file_target_size)) {
     return nullptr;
   }
   // if there is only one small file to merge, no need to perform