git-debrebase: Actually avoid read-tree in walk when not rewriting

git-debrebase: Actually avoid read-tree in walk when not rewriting

See
   git-debrebase: Prep for avoid read-tree in walk when not rewriting
for the motivation.

Here we try to check that the commit we would make, if we were
actually calling read-tree, write-tree and hash-object, would be
identical to the one we are reusing.

In particular, we check that the upstream files, and the debian/
directory, have the contents we would feed to read-tree.  And we check
that the parent list is the same.

With these checks this new algorithm is, I think, safe enough to just
use always.   So set $opt_careful to 0.  We will abolish this
variable in a moment.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>