forgit: add livecheck

By default, livecheck checks the Git tags for `forgit` and this works
but the formula's `stable` URL is a GitHub release asset and there
can be a notable gap between when a tag is created and a release
appears.

For example, 24.12.0 was tagged on 2024-11-30 but there hasn't been a
release yet, so livecheck has been returning this version as newest
even though we can't update to it until a GitHub release is available.

Formula/f/forgit.rb

@@ -5,6 +5,11 @@ class Forgit < Formula
   sha256 "7b78553561bf0201d7cf3d1c892af35044cf74d76079bcdc421113e178324ef5"
   license "MIT"
 
+  livecheck do
+    url :stable
+    strategy :github_latest
+  end
+
   bottle do
     sha256 cellar: :any_skip_relocation, all: "7a0752f36630c84cc3a7738fcc0928416022f38e10a56d54a127ae4ab748de3a"
   end