Fix parent rewriting in --early-output

Fix parent rewriting in --early-output

We cannot tell a node that has been checked and found not to be
interesting (which does not have the TREECHANGE flag) from a
node that hasn't been checked if it is interesting or not,
without relying on something else, such as object->parsed.

But an object can get the "parsed" flag for other reasons.
Which means that "TREECHANGE" has the wrong polarity.

This changes the way how the path pruning logic marks an
uninteresting commits.  From now on, we consider a commit
interesting by default, and explicitly mark the ones we decided
to prune.  The flag is renamed to "TREESAME".

Then, this fixes the logic to show the early output with
incomplete pruning.  It basically says "a commit that has
TREESAME set is kind-of-UNINTERESTING", but obviously in a
different way than an outright UNINTERESTING commit.  Until we
parse and examine enough parents to determine if a commit
becomes surely "kind-of-UNINTERESTING", we avoid rewriting
the ancestry so that later rounds can fix things up.

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