In upcoming patch we are going to switch xcommit_tree() to our own
implementation, and since this can potentially change how commits are
represented, for backward compatibility reason we need to make sure
objects encoded as commits stay the same.

So for all kind of objects (they are present in testdata/ repositories)
add checks that:

    - encode/decode is idempotent
    - encoding and decoding produces exactly expected sha1

One nice side effect of this is that we can now remove runtime
consistency check from tail of decoding. That check was there from the
beginning - from 6f237f22 (git-backup: Initial draft) mainly present
because there was no testsuite at that time. That check place is however
even not completely right - in case we somehow wrongly pulled an object
it has to be detected at pull time, not restore time. So that check was
checking only 1/2 of implementation - and not the main one - that
decoding does not mess up.

Since now we have proper testsuite and add encode/decode tests in this
patch, we can remove that partial runtime check. And even if decoding
messes something up, despite having it testsuited, it will be 100%
caught by restore process, because for an extracted repository, if
there is no some object which needs to be present in it, pack generation
for that repository will fail. So we can be safe with the removal.

Time for restoring kirr/slapos.git from lab.nexedi.com backup

before: 5.5s
after:  3.5s

( so much because there are ~ 500 tags in slapos.git and currently tag
  encoding is done with spawning separate subprocess per tag )