Make git-diff-tree indicate when it flushes

There are times when gitk needs to know that the commits it has sent
to git-diff-tree --stdin did not match, and it needs to know in a
timely fashion even if none of them match.  At the moment,
git-diff-tree outputs nothing for non-matching commits, so it is
impossible for gitk to distinguish between git-diff-tree being slow
and git-diff-tree saying no.

This makes git-diff-tree flush its output and echo back the
input line when it is not a valid-looking object name.  Gitk, or
other users of git-diff-tree --stdin, can use a blank line or
any other "marker line" to indicate that git-diff-tree has
processed all the commits on its input up to the echoed back
marker line, and any commits that have not been output do not
match.

[jc: re-done after a couple of back-and-forth discussion on the list.]

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

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