Add test for checking out a file from a commit

This works, we just didn't have a test for it.

pkg/integration/tests/test_list.go

@@ -89,6 +89,7 @@ var tests = []*components.IntegrationTest{
 	commit.AmendWhenThereAreConflictsAndContinue,
 	commit.AutoWrapMessage,
 	commit.Checkout,
+	commit.CheckoutFileFromCommit,
 	commit.Commit,
 	commit.CommitMultiline,
 	commit.CommitSkipHooks,