fix: return explicit message when changes() compares identical versions

[MohammadYusif]

Closes #2

## What

When `changes()` is called on two versions with identical content, the diff produces zero added and zero removed lines. Previously this rendered as:

system | v1 -> v2

• ADDED (0 lines)

• REMOVED (0 lines)

This is misleading — it looks like a real diff result rather than "nothing changed."

## Fix

Added an early-return check in `changes()` after filtering the diff lines. If both `added` and `removed` are empty, the method now returns:

No changes between v1 and v2.

## Test

Added `test_changes_identical_versions_returns_no_changes_message` in `tests/test_core.py`: saves the same content twice and asserts the exact "No changes between v1 and v2." message is returned.