Add test for checking out a file from a range selection of commits

The test shows a misbehavior: even though the diff shows "-one" and "+three",
meaning that "three" is the state we want to check out, we get "one". The reason
is that the checkout file command doesn't pay attention to range selections, it
only looks at the "moving end" of the range. Had we created the range by
selecting "two" and then pressed shift-up to "three", we would have gotten the
expected result.

pkg/integration/tests/test_list.go

@@ -90,6 +90,7 @@ var tests = []*components.IntegrationTest{
 	commit.AutoWrapMessage,
 	commit.Checkout,
 	commit.CheckoutFileFromCommit,
+	commit.CheckoutFileFromRangeSelectionOfCommits,
 	commit.Commit,
 	commit.CommitMultiline,
 	commit.CommitSkipHooks,