Fix Compare to return raw diff when source equals base

[ssam18]

Problem

When comparing a branch with an ancestor commit (where source == base), the CompareValueIterator was filtering out all changes that were 'added on destination', resulting in empty diff results.

This caused:

• lakectl branch revert to return 'no changes' error even when there were actual differences
• lakectl diff to return empty results when comparing with an ancestor tag/commit

Solution

Added a short-circuit in the Compare function: when source == base (i.e., comparing with an ancestor), return the raw diff iterator directly instead of wrapping it in CompareValueIterator which filters results.

When source equals base, the three-way comparison degenerates to a two-way diff, so the filtering logic is inappropriate.

Changes

• pkg/graveler/committed/manager.go: Added check for source == base case in Compare function
• pkg/graveler/committed/manager_test.go: Added TestManager_CompareSourceEqualsBase test

Testing

• All existing tests pass
• Added new test to verify the fix

Fixes #9802

[CLAassistant]

All committers have signed the CLA.

[arielshaqed]

Please see this comment on the issue. I believe you may be interested in the somewhat unfortunately-named two_dot comparison type in the OpenAPI spec.

Unfortunately, given that the existing API supports doing this in another way, and given that this would be a breaking API changing, this means that we will not be able to merge this PR.

Please free to discuss with us on #dev or #help on the lakeFS Slack workspace, lakefs.io/slack.