rev-list --verify-object

Often we want to verify everything reachable from a given set of commits
are present in our repository and connected without a gap to the tips of
our refs. We used to do this for this purpose:

$ rev-list --objects $commits_to_be_tested --not --all

Even though this is good enough for catching missing commits and trees,
we show the object name but do not verify their existence, let alone their
well-formedness, for the blob objects at the leaf level.

Add a new "--verify-object" option so that we can catch missing and broken
blobs as well.

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