fsck: default to "git fsck --full"

fsck: default to "git fsck --full"

Linus and other git developers from the early days trained their fingers
to type the command, every once in a while even without thinking, to check
the consistency of the repository back when the lower core part of the git
was still being developed.  Developers who wanted to make sure that git
correctly dealt with packfiles could deliberately trigger their creation
and checked them after they were created carefully, but loose objects are
the ones that are written by various commands from random codepaths.  It
made some technical sense to have a mode that checked only loose objects
from the debugging point of view for that reason.

Even for git developers, there no longer is any reason to type "git fsck"
every five minutes these days, worried that some newly created objects
might be corrupt due to recent change to git.

The reason we did not make "--full" the default is probably we trust our
filesystems a bit too much.  At least, we trusted filesystems more than we
trusted the lower core part of git that was under development.

Once a packfile is created and we always use it read-only, there didn't
seem to be much point in suspecting that the underlying filesystems or
disks may corrupt them in such a way that is not caught by the SHA-1
checksum over the entire packfile and per object checksum.  That trust in
the filesystems might have been a good tradeoff between fsck performance
and reliability on platforms git was initially developed on and for, but
it may not be true anymore as we run on many more platforms these days.

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