[rubygems/rubygems] Add tests for GitProxy#checkout

This commit adds tests to capture the current behavior of `#checkout`.
They are not exhaustive, but they cover cases cloning and fetching the
repository with different ref types. This will make it easier to change
the caching behavior in a subsequent commit.

https://github.com/rubygems/rubygems/commit/f637a412a6

spec/bundler/bundler/source/git/git_proxy_spec.rb

@@ -252,4 +252,75 @@ RSpec.describe Bundler::Source::Git::GitProxy do
       end
     end
   end
+
+  describe "#checkout" do
+    context "when the repository isn't cloned" do
+      before do
+        allow(path).to receive(:exist?).and_return(false)
+      end
+
+      it "clones the repository" do
+        allow(git_proxy).to receive(:git_local).with("--version").and_return("git version 2.14.0")
+        expect(git_proxy).to receive(:capture).with([*base_clone_args, "--", uri, path.to_s], nil).and_return(["", "", clone_result])
+        subject.checkout
+      end
+    end
+
+    context "when the repository is cloned" do
+      before do
+        allow(path).to receive(:exist?).and_return(true)
+      end
+
+      context "with a locked revision" do
+        let(:revision) { Digest::SHA1.hexdigest("ruby") }
+
+        context "when the revision exists locally" do
+          it "uses the cached revision" do
+            allow(git_proxy).to receive(:git_local).with("--version").and_return("git version 2.14.0")
+            expect(git_proxy).to receive(:git).with("cat-file", "-e", revision, dir: path).and_return(true)
+            subject.checkout
+          end
+        end
+
+        context "when the revision doesn't exist locally" do
+          it "fetches the specific revision" do
+            allow(git_proxy).to receive(:git_local).with("--version").and_return("git version 2.14.0")
+            expect(git_proxy).to receive(:git).with("cat-file", "-e", revision, dir: path).and_raise(Bundler::GitError)
+            expect(git_proxy).to receive(:capture).with(["fetch", "--force", "--quiet", "--no-tags", "--depth", "1", "--", uri, "#{revision}:refs/#{revision}-sha"], path).and_return(["", "", clone_result])
+            subject.checkout
+          end
+        end
+      end
+
+      context "with no explicit ref" do
+        it "fetches the HEAD revision" do
+          parsed_revision = Digest::SHA1.hexdigest("ruby")
+          allow(git_proxy).to receive(:git_local).with("rev-parse", "--abbrev-ref", "HEAD", dir: path).and_return(parsed_revision)
+          allow(git_proxy).to receive(:git_local).with("--version").and_return("git version 2.14.0")
+          expect(git_proxy).to receive(:capture).with(["fetch", "--force", "--quiet", "--no-tags", "--depth", "1", "--", uri, "refs/heads/#{parsed_revision}:refs/heads/#{parsed_revision}"], path).and_return(["", "", clone_result])
+          subject.checkout
+        end
+      end
+
+      context "with a commit ref" do
+        let(:ref) { Digest::SHA1.hexdigest("ruby") }
+
+        it "fetches the specific revision" do
+          allow(git_proxy).to receive(:git_local).with("--version").and_return("git version 2.14.0")
+          expect(git_proxy).to receive(:capture).with(["fetch", "--force", "--quiet", "--no-tags", "--depth", "1", "--", uri, "#{ref}:refs/#{ref}-sha"], path).and_return(["", "", clone_result])
+          subject.checkout
+        end
+      end
+
+      context "with a non-commit ref" do
+        let(:ref) { "HEAD" }
+
+        it "fetches all revisions" do
+          allow(git_proxy).to receive(:git_local).with("--version").and_return("git version 2.14.0")
+          expect(git_proxy).to receive(:capture).with(["fetch", "--force", "--quiet", "--no-tags", "--", uri, "refs/*:refs/*"], path).and_return(["", "", clone_result])
+          subject.checkout
+        end
+      end
+    end
+  end
 end