git-svn: use separate, per-repository .rev_db files

git-svn: use separate, per-repository .rev_db files

We need a separate .rev_db file for each repository we're
tracking.  This allows us to track the same logical path off
multiple mirrors.  We preserve a symlink to the old .rev_db
(no-UUID) if we're (auto-)migrating from an old version to
preserve backwards compatibility.

Also, get rid of the uuid() wrapper since we cache UUID in our
private config, and the SVN::Ra::get_uuid() function memoizes
the return value per-connection.

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