am -3: recover the diagnostic messages for corrupt patches

am -3: recover the diagnostic messages for corrupt patches

"git am -3" first tries to apply the patch without any extra trick, and
applies it to a synthesized tree for 3-way merge after the first attempt
fails.  "git apply" exits with status 1 for a patch that is well-formed
but is not applicable (and it dies on other errors with non-zereo, non-1
status) and has an optimization to fall back to the 3-way merge only in
the case.

An earlier patch 3ddd170 (am: suppress apply errors when using 3-way,
2009-06-16) squelched diagnostic messages from the first attempt, not to
be shown to the end user.  This worked reasonably well if the reason the
first application failed was because the patch was made against a wrong
version.

When the patch is corrupt (e.g. line-wrapped or leading whitespaces got
dropped), however, because the second patch application is not even
attempted, the error message from the first application is never shown
and is forever lost.  This message is necessary to locate where the patch
is corrupt and fix it up.

We could fix this issue by reverting 3dd170, or keeping the error message
to somewhere and showing it, but because this is an error codepath, the
easiest is to disable the optimization.  The second patch application is
attempted even when the input is corrupt, and it will notice, diagnose,
and stop with an error message.

Signed-off-by: Junio C Hamano <gitster@pobox.com>