There were some nice instructions for similar situations, but I could not find one for this disaster. I knew a reasonable amount about subversion before, now I know a lot more. First problem, the bifurcated repositories had a conflict. One file I had renamed, messed up the rename, and renamed it again in the next revision. I had also renamed it correctly in the other repository, so there was no chance that I would be able to just merge the repositories. Subversion made that very clear. I had to walk the original repository and find the offending revisions (1683, 1684). I was using the svnx GUI, so this was not too difficult. Here is the part that had to be precise. I had to dump the master repository in two pieces. Here is the code:
svnadmin dump MasterRepository -r 1:1682 > MasterRepository.svn.20140203_1.dump
svnadmin dump MasterRepository -r 1685:1751 --incremental > MasterRepository.svn.20140203_2.dump
Notice that the second dump is incremental. Maybe it could be done some other way, but this is the first thing that worked for me.
Next, dump just the revisions from the other repository which are since the split:
/usr/bin/svnadmin dump OtherDepository -r 1624:1721 --incremental > OtherDepository.dump
Now I need to move aside the original repository:
cd /Library/Server/Subversion
mv MasterRepository MasterRepository.old
Create the new repository on the server:
/usr/bin/svnadmin create MasterRepository
Load the three partial repositories, first the first dump of the Master, then the other two:
/usr/bin/svnadmin load MasterRepository < MasterRepository.svn.20140203_1.dump
/usr/bin/svnadmin load MasterRepository < OtherRepository.dump
/usr/bin/svnadmin load MasterRepository < MasterRepository.svn.20140203_2.dump
I still had problems with svnx not wanting to allow me to use my original directory as a working copy. I ended up moving it aside. Checking out a new version, and using tar to copy the old directory and paste it on top of the new directory.
tar cvf htdocs.tar htdocs
mv htdocs htdocs.old
(Here I used svnx to check out to working directory, which created directory, created .svn subdirectory, and copied all files in repository branch)
tar xvf htdocs.tar
Looking in svnx, the files which showed up as modified were exactly the right ones. There may be a more elegant way to accomplish this, but I am happy with the results.