diff: use "Diff Hunk" instead of "Diff Region"

Refer to diff regions as "Diff Hunk" throughout the
documentation and interface for consistency with other
patch/diff tools.

Closes #328

Suggested-by: Daniel Harding <dharding@living180.net>
Helped-by: Uri Okrent <uokrent@gmail.com>
Helped-by: Ｖ字龍(Vdragon) <Vdragon.Taiwan@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>