The default output from "fsck" is often overwhelmed by informational
message on dangling objects, especially if you do not repack often, and a
real error can easily be buried.

Add "--no-dangling" option to omit them, and update the user manual to
demonstrate its use.

Based on a patch by Clemens Buchacher, but reverted the part to change
the default to --no-dangling, which is unsuitable for the first patch.
The usual three-step procedure to break the backward compatibility over
time needs to happen on top of this, if we were to go in that direction.

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