#2485 fix payloads mixed up for webhook

When repository contains a Slack type hook,
it changes original payload content.

This patch fixes it by using a local object to store
newly created Slack payload instead of assigning
back to the same variable.

.bra.toml

@@ -13,7 +13,7 @@ watch_dirs = [
 watch_exts = [".go"]
 build_delay = 1500
 cmds = [
-	["go", "install", "-v", "-race"], # sqlite redis memcache cert pam tidb
+	["go", "install", "-v", "-race"], # sqlite cert pam tidb
 	["go", "build", "-race"],
 	["./gogs", "web"]
 ]