Update git diff explanation in history.md

[johnschoneboom]

Clarified the explanation of the default option for 'git diff' and its behavior with staged files.

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The Exploring History lesson asserted that HEAD was the default option for 'git diff', which is only true if the staging area is empty. If there are staged changes, 'git diff' compares the working area to the staged area, and adding HEAD will give a different result.

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[erinmgraham]

Thank you for suggesting alternative text for the git diff explanation, I appreciate the thought you put into this.

After digging into this further, including revisiting issue #1080, I think the better approach is to simplify the text here and clarify git diff more explicitly in episode 04, where it’s introduced. This episode is intended to focus on history and the use of identifiers and trying to capture the full variability of the git diff command here muddles the message.

I’m going to close this PR and follow up with a new one that links back here so your contribution and the discussion aren’t lost :-)