git-svn: make the $GIT_DIR/svn/*/revs directory obsolete

git-svn: make the $GIT_DIR/svn/*/revs directory obsolete

This is a very intrusive change, so I've beefed up the tests
significantly.  Added 'full-test' a target to the Makefile,
to test different possible configurations.  This is intended
for maintainers only.  Users should only be concerned with
'test' succeeding.

We now have a very simple custom database format for handling
mapping of svn revisions => git commits.  Of course, we're
not really using it yet, either.

Also disabled automatic branch-finding on new trees for now.
It's too easily broken.  revisions_eq() function should be
helpful for branch detection.

Also removed an extra assertion in fetch_cmd() that wasn't
correctly done.  This bug was found by full-test.

Signed-off-by: Eric Wong <normalperson@yhbt.net>