Improve the n and p shortcuts

[Julow]

The n and p shortcuts currently increment and decrement an integer. This integer is used as the index of the hunk to show on the screen.

This is a bad user experience as the integer can be incremented above the number of hunks, which has no visible effect except that the next n will also have no visible effect.

The p shortcuts should not set a value equal or above the number of hunks.
The n shortcuts should not set a negative value.

[yokurang]

Hi, could I please be assigned this task?

Here is a screenshot as required:

On another note, after reading the code base, it seems like this issue is closely related to this issue as well: #6

I think it might be more efficient to assign these both to me so my fix to this issue will likely be a fix to the related issue above too.

Cheers,
yokurang

[panglesd]

Let this issue and #6 be assigned to you!

[panglesd]

Fixed by #26 !