git-svn: teach find-rev to find near matches

git-svn: teach find-rev to find near matches

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":

    git svn find-rev --before \
        r$(git --git-dir=/over/there.git svn find-rev HEAD)

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Eric Wong <normalperson@yhbt.net>