By limiting start revision of find_rev_before to max existing
revision. This avoids a long wait if you do
'git svn reset -r 9999999'. The linear search within the
contiguous revisions doesn't seem to be a problem.
[ew: expanded commit message]
Signed-off-by: Ben Jackson <ben@ben.com> Acked-by: Eric Wong <normalperson@yhbt.net>