fix: /diff shows changes since the most recent message, not the previous one

[nolanchic]

Problem

/diff (and the diff aider auto-shows after each commit) displayed changes accumulated since the previous message instead of the current one, once two or more messages had been processed in a session.

raw_cmd_diff used commit_before_message[-2] as the diff base. But commit_before_message is appended exactly once per top-level message (in init_before_message, before the message is processed), so:

• [-1] = head recorded before the most recent message (correct base for "diff since the last message")
• [-2] = head recorded before the previous message (what the code used)

The sibling method show_undo_hint already uses [-1], so the two disagreed.

There was a second, related defect on the same path: the len(...) < 2 fallback used current_head + "^", which only goes back a single commit. For a message that produced multiple commits (e.g. via reflections), that showed just the last reflection's diff rather than the whole message's changes.

Fix

aider/commands.py — raw_cmd_diff:

if not self.coder.commit_before_message:
    commit_before_message = current_head + "^"
else:
    commit_before_message = self.coder.commit_before_message[-1]

This matches show_undo_hint (base_coder.py), and only falls back to current_head + "^" when no message has been recorded yet (empty list).

Test plan

• Added test_cmd_diff_since_last_message_not_previous — builds three commits C0 -> C1 -> C2, sets commit_before_message = [C0, C1] (HEAD = C2), and asserts /diff produces the C1..C2 diff (not C0..C2). Fails on main, passes with the fix.
• pytest tests/basic/test_commands.py — 71 passed (incl. the existing test_cmd_diff).
• pytest tests/basic/ -k "diff or undo or commit" — 30 passed.
• pre-commit run --files aider/commands.py tests/basic/test_commands.py — isort / black / flake8 / codespell all pass.

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