Fix gpg signature generation for tag and simplify signed data

- Use tag object ID instead of head commit as git note object ID
- Use note object ID as signed data

bin/git-signatures

@@ -215,20 +215,8 @@ path_check() {
 	return 0
 }
 
-default_base() {
-	# are we signing the genesis commit?
-	git rev-parse HEAD^ 1>/dev/null 2>&1
-	if [ "$?" -eq 128 ]; then
-		echo "4b825dc642cb6eb9a060e54bf8d69288fbee4904";
-	else
-		echo "HEAD^"
-	fi
-}
-
 sig_subject() {
-	git -c diff.indentHeuristic=false diff-tree -p "$1"..HEAD \
-	| git patch-id --stable \
-	| cut -d' ' -f1
+  printf "%s" "$(git show-ref "$1" -s)"
 }
 
 sig_parse(){
@@ -267,19 +255,18 @@ sig_parse(){
 
 sig_decode() {
 	local sig="$1"
-	local base="$2"
+	local ref="$2"
 	gpg --verify --trustdb-name="$trust_db" --status-fd=1 \
 	<(printf '%s' "$sig" | openssl base64 -d -A) \
-	<(sig_subject "$base") 2>/dev/null
+	<(sig_subject "$ref") 2>/dev/null
 }
 
 get_sigs() {
 	local ref="$1"
-	local base="$2"
 	while IFS='' read -r line; do
 		# shellcheck disable=SC2005
 		# TODO: Figure out some other way to do this
-		echo "$(sig_parse "$(sig_decode "$line" "$base")")"
+		echo "$(sig_parse "$(sig_decode "$line" "$ref")")"
 	done < <(git notes --ref signatures show "$ref" | grep -v "^$")
 }
 
@@ -300,19 +287,15 @@ cmd_add() {
 	[ "$#" -gt 2 ] && usage add && exit 1
 
 	ref=${1:-HEAD}
-	base=${2:-$(default_base)}
 	key=${key_id:-$(git config user.signingKey)}
 	gpg --list-secret-keys "$key" &> /dev/null || {
 		error "invalid_private_key" "$key"; exit 1;
 	}
 
-	# check for HEAD equal to base
-	[ "$(git rev-parse "$base")" == "$(git rev-parse HEAD)" ] && echo "error: cannot sign empty diff" && exit 1
-
-	sig_subject "$base" \
+	sig_subject "$ref" \
 	| gpg --detach-sign --local-user "$key" \
 	| openssl base64 -A \
-	| git notes --ref signatures append --file=-
+	| git notes --ref signatures append --file=- "$ref"
 
 	TAG_TARGET=$(git rev-parse refs/notes/signatures 2>/dev/null) && git tag -f latest-signature "$TAG_TARGET"
 
@@ -329,9 +312,7 @@ cmd_show() {
 		-t|--trust-db) trust_db="$2"; shift 2;;
 		--) shift; break ;;
 	esac done
-	[ "$#" -gt 2 ] && usage show && exit 1
 	ref=${1:-HEAD}
-	base=${2:-$(default_base)}
 	if [ "$raw" -ne 1 ]; then
 		printf " %-16s | %-10s | %-9s | %-28s | %-50s \\n" \
 			"Public Key ID" "Status" "Trust" "Date" "Signer Name"
@@ -343,7 +324,7 @@ cmd_show() {
 		IFS="|" read -d '' -ra sig < <(echo -n "$sig_parsed")
 		printf " %-16s | %-10s | %-9s | %28s | %-50s\\n" \
 			"${sig[0]}" "${sig[1]}" "${sig[2]}" "${sig[3]}" "${sig[4]}"
-	done < <(get_sigs "$ref" "$base")
+	done < <(get_sigs "$ref")
 }
 
 cmd_verify() {
@@ -358,9 +339,8 @@ cmd_verify() {
 	esac done
 	[ "$#" -gt 2 ] && usage verify && exit 1
 	ref=${1:-HEAD}
-	base=${2:-$(default_base)}
 	valid_count=$( \
-		cmd_show --raw --trust-db="$trust_db" "$ref" "$base" \
+		cmd_show --raw --trust-db="$trust_db" "$ref" \
 		| grep "ULTIMATE" \
 		| awk -F"|" '{ print $1 }' \
 		| uniq \