git-fsck-objects: lacking default references should not be fatal

git-fsck-objects: lacking default references should not be fatal

The comment added says it all: if we have lost all references in a git
archive, git-fsck-objects should still work, so instead of dying it should
just notify the user about that condition.

This change was triggered by me just doing a "git-init-db" and then
populating that empty git archive with a pack/index file to look at it.
Having git-fsck-objects not work just because I didn't have any references
handy was rather irritating, since part of the reason for running
git-fsck-objects in the first place was to _find_ the missing references.

However, "--unreachable" really doesn't make sense in that situation, and
we want to turn it off to protect anybody who uses the old "git prune"
shell-script (rather than the modern built-in). The old pruning script
used to remove all objects that were reported as unreachable, and without
any refs, that obviously means everything - not worth it.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>