Add option to auto recenter after diff-hl-{next,previous}-hunk

[NicholasBHubbard]

Hello. This PR adds a new variable diff-hl-next-previous-hunk-auto-recenter, that when set causes diff-hl-next-hunk and diff-hl-previous-hunk to automatically call recenter after moving the point.

In my configuration I use after advice on these functions to do this auto-recentering, but I thought it would be nicer if there was a diff-hl variable for this behavior.

Let me know if you think this is a good idea, and if there is anything you would like changed in this PR. Thanks!

[dgutov]

Hi!

Could this approach be a little problematic?

This function is also called from diff-hl-stage-current-hunk and diff-hl-revert-hunk, where I'm not sure if recentering is good or not.