Rely on fast-import for information about previous revs.
This requires always setting up backward flow of information, even for
v2 dumps. On the plus side, it simplifies the code by quite a bit and
opens the door to further simplifications.
[db: adjusted to support final version of the cat-blob patch]
[jn: avoiding hard-coding git's name for the empty tree for
portability to other backends]
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
723b7a2789