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>

.githooks/pre-commit

@@ -65,7 +65,7 @@ is_symlink() {
 # available in git tree and checksums are correct.
 check_local_sources() {
 	local apkbuild="$1"
-	local startdir="${apkbuild%/*}"
+	local startdir="$2"
 	local status=0
 	local checksum_act checksum_exp content filename line sources
 
@@ -109,11 +109,21 @@ check_file_size() {
 	fi
 }
 
+cleanup() {
+  [ -f "$_staged" ] && rm "$_staged"
+}
+
+trap cleanup EXIT
 
 for apkbuild in $(changed_files '**/APKBUILD'); do
-	check_local_sources "$apkbuild"
+	_staged="$(mktemp)"
+	git show ":$apkbuild" >"$_staged"
+	check_local_sources "$_staged" "${apkbuild%/*}"
+	cleanup
 done
 
 for path in $(changed_files); do
 	check_file_size "$path"
 done
+
+trap - EXIT