Importing symbols from a loaded diff in IDA is significantly slower than a diff performed within IDA

Steps to reproduce the problem:

1. Load two large binaries and create IDBs
2. Bindiff one of them against the other
3. Import a couple of symbols and note the time it takes
4. Save the diff results
5. Restart IDA
6. Load saved diff results
7. Import the same number of symbols and again note the time it takes.

What is the expected behavior?

It should take about the same amount of time.

What went wrong?

I bindiffed bindiff (haha) against binexport and put breakpoints on calls to `BinExport2::BinExport2` and `google::protobuf::MessageLite::ParsePartialFromIstream`. When working with a fresh diff performed within IDA bindiff does not load the binexport file for every symbol, but when working with loaded diff results binexport will load the binexport files for EACH symbol that's being ported.

What version of the product are you using? On what operating system? Bindiff 7, windows x64, IDA 7.6.210427

Any other comments? No

Ported from [b/199001147](https://issuetracker.google.com/199001147)