The redirection of the standard error stream to a temporary file is
a leftover cruft during debugging. Remove it.
Besides, it is reported by folks on the Windows that the test is
flaky with this redirection; somebody gets confused and this
merely-redirected-to file gets marked as delete-pending by git.exe
and makes it finish with a non-zero exit status when "git checkout"
finishes. Windows folks may want to figure that one out, but for
the purpose of this test, it shouldn't become a show-stopper.