Improve memory usage of `concat_changes` method

[MarcelGeo]

Problem

When using the geodiff concat_changes method with a large diff file (e.g., 350 MB), the RAM usage increases unexpectedly to approximately 3 GB. This memory peak can lead to Out of Memory (OOM) errors in server deployments where container resources are limited.

@wonder-sk I sent you the problematic diff file in private channel.

[wonder-sk]

So the concat changes operation loads all the diffs into memory. And the in-memory representation of these test diffs is much bigger - it can be roughly ~5x more when diff file content is read into ChangesetEntry objects representation.