main: rename "Index" menu to "Stage"

For consistency.

Closes #283

Reported-by: Ｖ字龍(Vdragon) <pika1021@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
diff: replace "Section" references with "Diff Region"

The term "Section" is ambiguous to new users.
Use "Diff Region" since that is more explicit.

Closes #297

Suggested-by: Ｖ字龍(Vdragon) <pika1021@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>