gluon-autoupdater: factor out default_branch() function

Make the code clearer and prepare for invalid branch fixup.

package/gluon-autoupdater/luasrc/lib/gluon/upgrade/500-autoupdater

@@ -21,16 +21,21 @@ for name, config in pairs(site.autoupdater.branches()) do
 	end
 end
 
-if not uci:get('autoupdater', 'settings') then
-	local enabled = unistd.access('/lib/gluon/autoupdater/default_enabled') ~= nil
-
-	local branch = site.autoupdater.branch(min_branch)
+local function default_branch()
 	local f = io.open('/lib/gluon/autoupdater/default_branch')
 	if f then
-		branch = f:read('*line')
+		local ret = f:read('*line')
 		f:close()
+		return ret
 	end
 
+	return site.autoupdater.branch(min_branch)
+end
+
+if not uci:get('autoupdater', 'settings') then
+	local enabled = unistd.access('/lib/gluon/autoupdater/default_enabled') ~= nil
+
+	local branch = default_branch()
 	if not branch then
 		enabled = false
 	end