Reuse previous annotation when overwriting a tag

When forcing to overwrite an annotated tag, there are good chances one
wants to keep the old annotation, or modify it, not start from scratch.

This is obviously only triggered for annotated tagging (-a or -s).

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-tag.c

@@ -247,9 +247,37 @@ static int git_tag_config(const char *var, const char *value)
 	return git_default_config(var, value);
 }
 
+static void write_tag_body(int fd, const unsigned char *sha1)
+{
+	unsigned long size;
+	enum object_type type;
+	char *buf, *sp, *eob;
+	size_t len;
+
+	buf = read_sha1_file(sha1, &type, &size);
+	if (!buf)
+		return;
+	/* skip header */
+	sp = strstr(buf, "\n\n");
+
+	if (!sp || !size || type != OBJ_TAG) {
+		free(buf);
+		return;
+	}
+	sp += 2; /* skip the 2 LFs */
+	eob = strstr(sp, "\n" PGP_SIGNATURE "\n");
+	if (eob)
+		len = eob - sp;
+	else
+		len = buf + size - sp;
+	write_or_die(fd, sp, len);
+
+	free(buf);
+}
+
 static void create_tag(const unsigned char *object, const char *tag,
 		       struct strbuf *buf, int message, int sign,
-			   unsigned char *result)
+			   unsigned char *prev, unsigned char *result)
 {
 	enum object_type type;
 	char header_buf[1024];
@@ -282,7 +310,11 @@ static void create_tag(const unsigned char *object, const char *tag,
 		if (fd < 0)
 			die("could not create file '%s': %s",
 						path, strerror(errno));
-		write_or_die(fd, tag_template, strlen(tag_template));
+
+		if (!is_null_sha1(prev))
+			write_tag_body(fd, prev);
+		else
+			write_or_die(fd, tag_template, strlen(tag_template));
 		close(fd);
 
 		launch_editor(path, buf);
@@ -419,7 +451,7 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 		die("tag '%s' already exists", tag);
 
 	if (annotate)
-		create_tag(object, tag, &buf, message, sign, object);
+		create_tag(object, tag, &buf, message, sign, prev, object);
 
 	lock = lock_any_ref_for_update(ref, prev, 0);
 	if (!lock)