When a single SVN repository is split into multiple Git repositories
many SVN revisions will exist in only one of the Git repositories
created. For some projects the only way to build a working artifact is
to check out corresponding versions of various repositories, with no
indication of what those are in the Git world - in the SVN world the
revision numbers are sufficient.
By adding "--before" to "git-svn find-rev" we can say "tell me what this
repository looked like when that other repository looked like this":