git-svn: bugfix: allow SVN:: lib users to track the root of the repository

git-svn: bugfix: allow SVN:: lib users to track the root of the repository

I'm not sure if anybody has hit this (besides me), but this
fixes the problem where I ran into while attempting to import a
small repo at the root level:  I ended up with all the commits, but
with no file/tree changes at all throughout the entire history.

Also, fix a warning if the commit message is not defined for revision 0.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>