pr-pull: add checksum update clause

Library/Homebrew/dev-cmd/pr-pull.rb

@@ -144,6 +144,8 @@ module Homebrew
       "#{subject_name} #{new_package.version}"
     elsif !is_cask && old_package.revision != new_package.revision
       "#{subject_name}: revision #{reason}".strip
+    elsif is_cask && old_package.sha256 != new_package.sha256
+      "#{subject_name}: checksum update #{reason}".strip
     else
       "#{subject_name}: #{reason || "rebuild"}".strip
     end

Library/Homebrew/test/dev-cmd/pr-pull_spec.rb

@@ -45,6 +45,17 @@ describe "brew pr-pull" do
         cask "food" do
           desc "Helpful description"
           version "1.0"
+          sha256 "a"
+          url "https://brew.sh/food-\#{version}.tgz"
+        end
+      EOS
+    end
+    let(:cask_checksum) do
+      <<~EOS
+        cask "food" do
+          desc "Helpful description"
+          version "1.0"
+          sha256 "b"
           url "https://brew.sh/food-\#{version}.tgz"
         end
       EOS
@@ -53,6 +64,7 @@ describe "brew pr-pull" do
       <<~EOS
         cask "food" do
           version "2.0"
+          sha256 "a"
           url "https://brew.sh/food-\#{version}.tgz"
         end
       EOS
@@ -61,6 +73,7 @@ describe "brew pr-pull" do
       <<~EOS
         cask "food" do
           version "1.0"
+          sha256 "a"
           url "https://brew.sh/food-\#{version}.tgz"
         end
       EOS
@@ -164,6 +177,10 @@ describe "brew pr-pull" do
         expect(described_class.determine_bump_subject(cask, cask_version, cask_file)).to eq("food 2.0")
       end
 
+      it "correctly bumps a cask checksum" do
+        expect(described_class.determine_bump_subject(cask, cask_checksum, cask_file)).to eq("food: checksum update")
+      end
+
       it "correctly bumps a formula revision with reason" do
         expect(described_class.determine_bump_subject(
                  formula, formula_revision, formula_file, reason: "for fun"