utils/git: new file_at_commit function

Library/Homebrew/test/utils/git_spec.rb

@@ -68,6 +68,17 @@ describe Utils::Git do
     end
   end
 
+  describe "#file_at_commit" do
+    it "returns file contents when file exists" do
+      expect(described_class.file_at_commit(HOMEBREW_CACHE, file, file_hash1)).to eq("README")
+    end
+
+    it "returns empty when file doesn't exist" do
+      expect(described_class.file_at_commit(HOMEBREW_CACHE, "foo.txt", file_hash1)).to eq("")
+      expect(described_class.file_at_commit(HOMEBREW_CACHE, "LICENSE.txt", file_hash1)).to eq("")
+    end
+  end
+
   describe "#last_revision_commit_of_files" do
     context "when before_commit is nil" do
       it "gives last revision commit" do

Library/Homebrew/utils/git.rb

@@ -76,9 +76,14 @@ module Utils
 
     def last_revision_of_file(repo, file, before_commit: nil)
       relative_file = Pathname(file).relative_path_from(repo)
-
       commit_hash = last_revision_commit_of_file(repo, relative_file, before_commit: before_commit)
-      Utils.popen_read(git, "-C", repo, "show", "#{commit_hash}:#{relative_file}")
+      file_at_commit(repo, file, commit_hash)
+    end
+
+    def file_at_commit(repo, file, commit)
+      relative_file = Pathname(file)
+      relative_file = relative_file.relative_path_from(repo) if relative_file.absolute?
+      Utils.popen_read(git, "-C", repo, "show", "#{commit}:#{relative_file}")
     end
 
     def ensure_installed!