clarify git diff behavior when HEAD is omitted

[erinmgraham]

Maybe related, I stumbled upon the following sentence today:

Note that HEAD is the default option for git diff, so omitting it will not change the command’s output at all (give it a try).

I found this to be very confusing, because the output is only the same if no additional files have been staged since the last commit.

_Originally posted by @fkohrt in #769

[erinmgraham]

Hi @fkohrt, thank you for your contribution! I will make a new issue with your comment because the text you referenced is found in episode 05-history and issue 769 relates to episode 04-changes.

Are you able to suggest alternative text that would be less confusing?

[fkohrt]

Maybe as follows?

If run without arguments, git diff compares the files in your working directory with what is about to be save permanently (the staging area). git diff HEAD compares the files in your working directory with the most recently committed version instead. The real power of git diff lies in its ability to compare with previous commits.

Displaying this comparison in another diagram like the one by @astroDimitrios here might additionaly be helpful.