githooks: fix checksum comparison in pre-commit hook

The previous method didn't work for larger files.

.githooks/pre-commit

@@ -57,22 +57,23 @@ check_local_sources() {
 	local apkbuild="$1"
 	local startdir="${apkbuild%/*}"
 	local status=0
-	local checksum content filename line sources
+	local checksum_act checksum_exp content filename line sources
 
 	sources=$(abuild_local_sources "$apkbuild")
 	for line in $sources; do
 		filename=${line%%:*}
-		checksum=${line#*:}
+		checksum_exp=${line#*:}
 
-		content=$(git show ":$startdir/$filename" 2>/dev/null) || {
+		if ! git cat-file -e ":$startdir/$filename" 2>/dev/null; then
 			error "$startdir: missing file \"$filename\""
 			status=1
 			continue
-		}
-		[ "$(printf '%s\n' "$content" | sha512sum)" = "$checksum  -" ] || {
+		fi
+		checksum_act=$(git show ":$startdir/$filename" | sha512sum)
+		if [ "$checksum_act" != "$checksum_exp  -" ]; then
 			error "$startdir: bad checksum for file \"$filename\" (hint: run abuild checksum)"
 			status=1
-		}
+		fi
 	done
 
 	return $status