contrib/git-svn: allow rebuild to work on non-linear remote heads

Because committing back to an SVN repository from different
machines can result in different lineages, two different
repositories running git-svn can result in different commit
SHA1s (but of the same tree).  Sometimes trees that are tracked
independently are merged together (usually via children),
resulting in non-unique git-svn-id: lines in rev-list.

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