format-patch: threading test reactivation

format-patch: threading test reactivation

t4014 tests format-patch --thread since 7d812145, but the tests were
ineffective right from the start at least for bash and dash.  The
loops of the form

  for ...; do something || break; done

introduced by 7d812145 and 5d02294 always exit with status 0, even if
'something' failed, because 'break' returns 0 unless there was no loop
to break.

We take a rather different approach that uses an admittedly heinous
inline Perl script to mangle all interesting information into a format
that is invariant between runs.  We can then test the full patch
sequence in one go (with --stdout), doing away with the loop problem.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>