vcs-svn: eliminate repo_tree structure

vcs-svn: eliminate repo_tree structure

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 <redacted>
Signed-off-by: David Barr <redacted>
Signed-off-by: Jonathan Nieder <redacted>