Tldr: git diff: add relative time ranges examples

[waldyrious]

git diff 'HEAD@{yesterday morning}'
git diff 'HEAD@{3 days ago}'
git diff 'HEAD@{one week ago}'

and

git diff --since="yesterday morning"
git diff --since="3 days ago"
git diff --since="one week ago"

From this reddit thread. Not sure exactly what the difference is between the two syntaxes, I'd need to investigate a bit. If anyone knows, please weigh in below!

For reference, here's the current git-diff.md page.

[mdew192837]

Here may be something useful. I'll take a look at this though.

[jitakirin]

BTW, I didn't have much luck with the --since= syntax with diff, it gives me empty results for any expression I tried. git rev-parse --since='4 days ago' gives --max-age=1509142638, so this works very well with log. For comparison git rev-parse 'HEAD@{4 days ago}' gives an actual commit ID.