feat: Generate shell completions on install

Homebrew can automatically generate and install completions for bash,
zsh, and fish. For now, completion is limited to bash and zsh until
graphite adds fish support.

This simplifies the installation steps for homebrew users.

Formula/graphite.rb

@@ -3,16 +3,18 @@
 class Graphite < Formula
   desc "The Graphite CLI allows you to easily manage your stacked-diff workflow."
   homepage "https://graphite.dev/"
-  url "https://github.com/withgraphite/graphite-cli/archive/v0.20.11.zip"
-  sha256 "2458a015ba46c636b138fec424cd26ffa09430369f5420672cb14329147233d0"
-  version "0.20.11"
+  url "https://github.com/withgraphite/graphite-cli/archive/v0.20.13.zip"
+  sha256 "e5bbc20efa6838af2f78413d8cfc44957500af48b96d310faea49cda784e9207"
+  version "0.20.13"
   license "None"
 
   depends_on "node@16"
 
   def install
     system "npm", "install", *Language::Node.std_npm_install_args(libexec)
     bin.install_symlink Dir["#{libexec}/bin/*"]
+
+    generate_completions_from_executable(libexec/"bin/gt", "completion", shells: [:bash, :zsh])
   end
 
   test do

formula-templates/graphite.rb

@@ -13,6 +13,8 @@ class Graphite < Formula
   def install
     system "npm", "install", *Language::Node.std_npm_install_args(libexec)
     bin.install_symlink Dir["#{libexec}/bin/*"]
+
+    generate_completions_from_executable(libexec/"bin/gt", "completion", shells: [:bash, :zsh])
   end
 
   test do