tag: use strbuf to format tag header

We format the tag header into a fixed 1024-byte buffer. But
since the tag-name and tagger ident can be arbitrarily
large, we may unceremoniously die with "tag header too big".
Let's just use a strbuf instead.

Note that it looks at first glance like we can just format
this directly into the "buf" strbuf where it will ultimately
go. But that buffer may already contain the tag message, and
we have no easy way to prepend formatted data to a strbuf
(we can only splice in an already-generated buffer). This
isn't a performance-critical path, so going through an extra
buffer isn't a big deal.

Signed-off-by: Jeff King <peff@peff.net>

builtin/tag.c

@@ -231,15 +231,14 @@ static void create_tag(const unsigned char *object, const char *tag,
 		       unsigned char *prev, unsigned char *result)
 {
 	enum object_type type;
-	char header_buf[1024];
-	int header_len;
+	struct strbuf header = STRBUF_INIT;
 	char *path = NULL;
 
 	type = sha1_object_info(object, NULL);
 	if (type <= OBJ_NONE)
 	    die(_("bad object type."));
 
-	header_len = snprintf(header_buf, sizeof(header_buf),
+	strbuf_addf(&header,
 		    "object %s\n"
 		    "type %s\n"
 		    "tag %s\n"
@@ -249,9 +248,6 @@ static void create_tag(const unsigned char *object, const char *tag,
 		    tag,
 		    git_committer_info(IDENT_STRICT));
 
-	if (header_len > sizeof(header_buf) - 1)
-		die(_("tag header too big."));
-
 	if (!opt->message_given) {
 		int fd;
 
@@ -288,7 +284,8 @@ static void create_tag(const unsigned char *object, const char *tag,
 	if (!opt->message_given && !buf->len)
 		die(_("no tag message?"));
 
-	strbuf_insert(buf, 0, header_buf, header_len);
+	strbuf_insert(buf, 0, header.buf, header.len);
+	strbuf_release(&header);
 
 	if (build_tag_object(buf, opt->sign, result) < 0) {
 		if (path)