Be more specific about HEAD^ vs HEAD~

[Tatsujinichi]

The text as it is kind of implies that HEAD^2 would be equivalent to HEAD~2. I think it it worth adding a little more detail here.

[RichardLitt]

Fair. I think this needs a little bit more - can you add a sentence showing that HEAD~2 points to the first parent's first parent, while HEAD^2 specifies the first parent's second parent?

More on this: http://www.paulboxley.com/blog/2011/06/git-caret-and-tilde

[Tatsujinichi]

My impression is that this document is not a substitute for the official documentation. But should be
a sparse cheat sheet with short solutions to common problems. Therefore I should not be overly verbose. I think this is already on the borderline of saying too much.

[RichardLitt]

Hmm. Fair. @k88hudson What do you think?

[drj11]

Now I've learnt (thanks @RichardLitt ) about the difference between thing^ and thing~ I think that it's never worth teaching novices HEAD^ and I should only ever use HEAD~.

In particular: HEAD~~ is the same as HEAD~2 and is often useful (the grandparent). HEAD^^ (grandparent) is not the same as HEAD^2 (second parent when it exists) and I think the difference will only confuse novices.

[k88hudson]

@Tatsujinichi @RichardLitt Personally, I wouldn't go with an extended explanation with examples here, I'd just briefly mention that HEAD~2 is not equivalent to HEAD^2. Something like:

This stands for the first parent of `HEAD`, similarly `HEAD^2` stands for the
second parent of the commit (merges can have 2 parents).
Note that `HEAD^2` is **not** the same as `HEAD~2`
(see [this link](http:....) for more information).

Even without the link, I think that would give folks enough context for further googling if necessary.

[RichardLitt]

Thanks @Tatsujinichi!