unpack-objects desperately salvages objects from a corrupt pack

unpack-objects desperately salvages objects from a corrupt pack

The command unpack-objects dies upon the first error.  This is
probably considered a feature -- if a pack is corrupt, instead
of trying to extract from it and possibly risking to contaminate
a good repository with objects whose validity is dubious, we
should seek a good copy of the pack and retry.  However, we may
not have any good copy anywhere.  This implements the last
resort effort to extract what are salvageable from such a
corrupt pack.

This flag might have helped Sergio when recovering from a
corrupt pack.  In my test, it managed to salvage 247 objects out
of a pack that had 251 objects but without it the command
stopped after extracting 73 objects.

Signed-off-by: Junio C Hamano <junkio@cox.net>