githooks: ignore unstaged hunks of APKBUILD (MR 5807)

It's possible that additional sources have been made to an APKBUILD
which are unstaged, but if the hunks adding those sources haven't been
staged either then it doesn't make sense to fail on this.

Use git show to get only the staged hunks for the APKBUILD.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>