receive-pack: factor out capability string generation

Similar to the previous one for send-pack, make it easier and
cleaner to add to capability advertisement.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/receive-pack.c

@@ -137,15 +137,21 @@ static void show_ref(const char *path, const unsigned char *sha1)
 	if (ref_is_hidden(path))
 		return;
 
-	if (sent_capabilities)
+	if (sent_capabilities) {
 		packet_write(1, "%s %s\n", sha1_to_hex(sha1), path);
-	else
-		packet_write(1, "%s %s%c%s%s agent=%s\n",
-			     sha1_to_hex(sha1), path, 0,
-			     " report-status delete-refs side-band-64k quiet",
-			     prefer_ofs_delta ? " ofs-delta" : "",
-			     git_user_agent_sanitized());
-	sent_capabilities = 1;
+	} else {
+		struct strbuf cap = STRBUF_INIT;
+
+		strbuf_addstr(&cap,
+			      "report-status delete-refs side-band-64k quiet");
+		if (prefer_ofs_delta)
+			strbuf_addstr(&cap, " ofs-delta");
+		strbuf_addf(&cap, " agent=%s", git_user_agent_sanitized());
+		packet_write(1, "%s %s%c%s\n",
+			     sha1_to_hex(sha1), path, 0, cap.buf);
+		strbuf_release(&cap);
+		sent_capabilities = 1;
+	}
 }
 
 static int show_ref_cb(const char *path, const unsigned char *sha1, int flag, void *unused)