git-debrebase: status: Reorganise commit reporting.

Replace the references to @_ with a hash.  Using a hash is much less
confusing than all this numerical indexing.  Also, \@_ is wrong
because there is only one @_, so by the time it is dereference, the
commit being printed is the anchor.

This is why in #903131 we see

      branch is unlaundered
      found mixed upstream/packaging commit (7badba627162337c7057002f37e9a1a593d07d08)
        abcc6260 Update to upstream 2.9.7

which appears to sugggest that the anchor is a mixed commit.  Actually
the mixed commit is 7badba627162 but the subsequent message is based
on @_ which has different information in it by then.

And, set $kcmsg from the main message, not including the commit
information.  We are the more sophisticated caller mentioned in the
previous commmit.

Closes: #903131.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>