apply: correctly reverse patch's pre- and post-image mode bits

apply: correctly reverse patch's pre- and post-image mode bits

When parsing the patch header, unless it is a patch that changes
file modes, we only read the mode bits into the .old_mode member of
the patch structure and leave .new_mode member as initialized, i.e.,
to 0.  Later when we need the original mode bits, we consult .old_mode.

However, reverse_patches() that is used to swap the names and modes
of the preimage and postimage files is not aware of this convention,
leading the .old_mode to be 0 while the mode we read from the patch
is left in .new_mode.

Only swap .old_mode and .new_mode when .new_mode is not 0 (i.e. we
saw a patch that modifies the filemode and know what the new mode
is).  When .new_mode is set to 0, it means the preimage and the
postimage files have the same mode (which is in the .old_mode member)
and when applying such a patch in reverse, the value in .old_mode is
what we expect the (reverse-) preimage file to have.

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