Improve on the 'invalid object' error message at commit time

Improve on the 'invalid object' error message at commit time

Not that anybody should ever get it, but somebody did (probably because
of a flaky filesystem, but whatever).  And each time I see an error
message that I haven't seen before, I decide that next time it will look
better.

So this makes us write more relevant information about exactly which
file ended up having issues with a missing object.  Which will tell
whether it was a tree object, for example, or just a regular file in the
index (and which one).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>