format-patch: avoid freopen()

format-patch: avoid freopen()

We just taught the relevant functions to respect the diffopt.file field,
to allow writing somewhere else than stdout. Let's make use of it.

Technically, we do not need to avoid that call in a builtin: we assume
that builtins (as opposed to library functions) are stand-alone programs
that may do with their (global) state. Yet, we want to be able to reuse
that code in properly lib-ified code, e.g. when converting scripts into
builtins.

Further, while we did not *have* to touch the cmd_show() and cmd_cherry()
code paths (because they do not want to write anywhere but stdout as of
yet), it just makes sense to be consistent, making it easier and safer to
move the code later.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>